Re: Any sailplane pilots?

[Larry Dighera]

On Sun, 21 Dec 2003 06:00:30 GMT, "Chuck" > wrote
in Message-Id: et>:

>My cousin just bought a Schweizer 1 36 sailplane.

I've some experience with the Schweizer 1-26. It had an L/D of <30
IIRC. Do you know the L/D of the 1-36?
http://www.dfrc.nasa.gov/Gallery/Photo/Schweizer-1-36/

>We took it to the airport today where he is going to hanger it today, and
>put her together so the FAA could give the stamp of approval with the
>airworthiness certificate.
>
>Looks like the sailplane could be alot of fun.

It is a contest between the pilot and mother nature. The idea is to
spend more time in rising air than sinking air, and thus gain and
sustain altitude. The pilot must mentally visualize the movements of
the air masses in his vicinity, due to convective and orographic
vertical displacement, solely through interpreting instrument
indications and seat-of-the-pants cues.

The spectrum of soaring meteorological conditions ranges from flat
(little or no vertical movement of the air mass) to booming. During
the latter, the pilot is nearly unable to prevent his ship from
rising; it's like having a motor. On an average day, a pilot will
spend a great deal of time attempting to "core" thermals. That
involves mentally visualizing the sailplane's position relative to the
thermal's vertical anticyclone column center, and guiding his
sailplane to circle as near to the center of it as he is able.
Because this can require banks in excess of 60 degrees, occupants ware
a parachute. Such long, constant high-G circling can adversely affect
passengers of multi-place sailplanes, but the pilot will find it
exhilarating. The sport of soaring adds another dimension to similar
naturally powered sports such as sailing, surfing, and skiing...

>I have never been around them before.
>
>Just wondering how many guys fly gliders...
>

[newsgroup rec.aviation.soaring added]

[Eric Greenwell]

Larry Dighera wrote:


> The spectrum of soaring meteorological conditions ranges from flat
> (little or no vertical movement of the air mass) to booming. During
> the latter, the pilot is nearly unable to prevent his ship from
> rising; it's like having a motor. On an average day, a pilot will
> spend a great deal of time attempting to "core" thermals. That
> involves mentally visualizing the sailplane's position relative to the
> thermal's vertical anticyclone column center, and guiding his
> sailplane to circle as near to the center of it as he is able.
> Because this can require banks in excess of 60 degrees, occupants ware
> a parachute. Such long, constant high-G circling can adversely affect
> passengers of multi-place sailplanes, but the pilot will find it
> exhilarating. The sport of soaring adds another dimension to similar
> naturally powered sports such as sailing, surfing, and skiing...

Just in case you might get the idea from Larry that we all have our
blood at our feet from G loads, note that _most_ us use more like a 30
degree bank (g load hardly noticeable after a few flights), and can go
up just fine!

And for _most_ of us, wearing a parachute doesn't have anything to do
with circling in thermals. They aren't required except in contests, but
most private owners end up with one for various reasons, and wear it all
the time (sort of like wearing a seat belt in a car). Clubs aren't so
likely to use them while flying their gliders.

--
-----
change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

[K.P. Termaat]

Larry wrote:

>On an average day, a pilot will
> spend a great deal of time attempting to "core" thermals. That
> involves mentally visualizing the sailplane's position relative to the
> thermal's vertical anticyclone column center, and guiding his
> sailplane to circle as near to the center of it as he is able.

Why do you write "anticyclone" Larry. This would mean that all gliders would
circle righthanded to stay as close to the core as possible thereby using
the headwind of the thermal.
I have never found a meteorological reason to circle in a certain direction.
Most pilots circle lefthanded just because they were tought to do so in the
vicinity of their home field. However in x-country flights there is usually
no preference. In low situations the pilot usually circles in the direction
he feels most comfortable. Sometimes the gliders feels better when circling
to the right rather then to the left is my experience.

Karel, NL

"Eric Greenwell" > schreef in bericht
...
> Larry Dighera wrote:
>
>
> > The spectrum of soaring meteorological conditions ranges from flat
> > (little or no vertical movement of the air mass) to booming.
>
> Eric Greenwell
> Washington State
> USA
>

[Robin Birch]

In message >, Eric Greenwell
> writes
>Larry Dighera wrote:
>
>
>> The spectrum of soaring meteorological conditions ranges from flat
>> (little or no vertical movement of the air mass) to booming. During
>> the latter, the pilot is nearly unable to prevent his ship from
>> rising; it's like having a motor. On an average day, a pilot will
>> spend a great deal of time attempting to "core" thermals. That
>> involves mentally visualizing the sailplane's position relative to the
>> thermal's vertical anticyclone column center, and guiding his
>> sailplane to circle as near to the center of it as he is able.
>> Because this can require banks in excess of 60 degrees, occupants ware
>> a parachute. Such long, constant high-G circling can adversely affect
>> passengers of multi-place sailplanes, but the pilot will find it
>> exhilarating. The sport of soaring adds another dimension to similar
>> naturally powered sports such as sailing, surfing, and skiing...
>
>Just in case you might get the idea from Larry that we all have our
>blood at our feet from G loads, note that _most_ us use more like a 30
>degree bank (g load hardly noticeable after a few flights), and can go
>up just fine!
>
>And for _most_ of us, wearing a parachute doesn't have anything to do
>with circling in thermals. They aren't required except in contests, but
>most private owners end up with one for various reasons, and wear it
>all the time (sort of like wearing a seat belt in a car). Clubs aren't
>so likely to use them while flying their gliders.
>
Don't know about the states but we use parachutes in everything. As far
as I know everybody does in the UK.

Robin
--
Robin Birch

[Larry Dighera]

On Fri, 26 Dec 2003 09:00:24 GMT, "K.P. Termaat" > wrote
in Message-Id: >:

>Larry wrote:
>
>>On an average day, a pilot will
>> spend a great deal of time attempting to "core" thermals. That
>> involves mentally visualizing the sailplane's position relative to the
>> thermal's vertical anticyclone column center, and guiding his
>> sailplane to circle as near to the center of it as he is able.
>
>Why do you write "anticyclone" Larry.

That was my recollection from my soaring experience in the early '70s.
However, it appears that the word 'cyclone' may have more correct now
that I have done some research:

The National Weather Service Glossary page here
http://www.wrh.noaa.gov/Phoenix/general/glossary/ has this to say:

ANTICYCLONE - A region of high atmospheric pressure around which
winds move in a clockwise direction in the northern hemisphere.
Generally associated with warm and dry weather

CYCLONE - An area of low pressure around which winds blow in a
counter-clockwise fashion in the northern hemisphere. Generally
associated with cool, wet and unsettled weather

On the other hand ...

>This would mean that all gliders would circle righthanded to stay as close
>to the core as possible thereby using the headwind of the thermal.

I'll defer to these sources:

http://www.rc-soar.com/tech/thermals.htm
Do thermals rotate?
They do, but not predictably. Even dust devils don't have a
preferred direction of rotation (see Stull, p.449). Thermals are
too small and too short-lived to be affected by the earth's
rotation (Coriolis force) or by the equator/pole thermal gradient.
Their rotation is determined by local terrain. Rotational velocity
in the core of a typical thermal is small compared to the vertical
velocity.


http://www.skynomad.com/articles/athermal10.html
THERMALS

Do Thermals Spin?
by Peter Gray

Of course, dust devils spin, so thermals probably do also, at
least when they're close to the ground. For many years, I was
convinced that dust devils rotated in random directions. However,
based on a few flights in northern Washington State, where I kept
more careful mental notes, I would guess that about 2/3 to 3/4 of
the dust devils there spin counterclockwise, in agreement with the
Coriolis Effect. Contrary to popular belief, the smaller-scale
equivalent, water going down a toilet or drain, is essentially
unaffected, and the rotation, if it is biased toward one spin
direction, is the result of the geometry of each such basin (check
it out for yourself!).

The dust devils formed by thermals seem to be just large
enough to be affected somewhat by the Coriolis Effect. If I have
the choice, I usually opt for circling against the dust devil's
rotation, most often to the right, in case this will produce a
better climb rate by reducing my circling ground speed, thus bank
angle and sinkrate.

However, when I have reversed direction several times in one
climb, I have rarely detected a significant advantage in one
direction. What little angular momentum thermals start with, they
seem to lose through drag in the surrounding air, and they
probably start with little spin anyway. As with water going down a
drain, very little spin momentum at a large radius can translate
to a rapid spin when the radius shrinks to that of a dust devil.

>I have never found a meteorological reason to circle in a certain direction.
>Most pilots circle lefthanded just because they were tought to do so in the
>vicinity of their home field. However in x-country flights there is usually
>no preference. In low situations the pilot usually circles in the direction
>he feels most comfortable. Sometimes the gliders feels better when circling
>to the right rather then to the left is my experience.
>
>Karel, NL

Agreed. Circling direction is more often dictated by other gliders in
the thermal than meteorological phenomena and physics.

[Mike Lindsay]

In article >, Robin Birch <robinb@r
>Don't know about the states but we use parachutes in everything. As far
>as I know everybody does in the UK.
>
>Robin

Everything except driving the tug.
--
Mike Lindsay

[K.P. Termaat]

Thanks Larry for your interesting respons with your links to the articles.

As a matter of fact most members of our National Team (my son is one of
them) tend to circle to the right when low. They don't in effect know why.
They say, and I agree, that it usually feels better in getting up again. So
Peter Gray may be right in saying that most of the thermals have the
tendency to rotate to the left in the northern hemisphere, especially when
low when still having small diameters. Coriolis may be the cause then. Would
be interesting to know the experience in the southern hemisphere.

Talking about a drain and water brings me to the idea of telling that when
low and looking for a thermal I always try to locate small ponds in dry
areas. It looks to me that the water vapor rising from these ponds is an
excellent means of starting a thermal. Water vapor is lighter then air, so
it increases the boyancy of the air over the pond and off it goes. Starting
at about ground level, circling to the right may then generally be the
better option. Any experience with this Larry?

Karel, NL


"Larry Dighera" > schreef in bericht
...
> On Fri, 26 Dec 2003 09:00:24 GMT, "K.P. Termaat" > wrote
> in Message-Id: >:
>
> >Larry wrote:
> >
> >>On an average day, a pilot will
> >> spend a great deal of time attempting to "core" thermals. That
> >> involves mentally visualizing the sailplane's position relative to the
> >> thermal's vertical anticyclone column center, and guiding his
> >> sailplane to circle as near to the center of it as he is able.
> >
> >Why do you write "anticyclone" Larry.
>
> That was my recollection from my soaring experience in the early '70s.
> However, it appears that the word 'cyclone' may have more correct now
> that I have done some research:
>
> The National Weather Service Glossary page here
> http://www.wrh.noaa.gov/Phoenix/general/glossary/ has this to say:
>
> ANTICYCLONE - A region of high atmospheric pressure around which
> winds move in a clockwise direction in the northern hemisphere.
> Generally associated with warm and dry weather
>
> CYCLONE - An area of low pressure around which winds blow in a
> counter-clockwise fashion in the northern hemisphere. Generally
> associated with cool, wet and unsettled weather
>
> On the other hand ...
>
> >This would mean that all gliders would circle righthanded to stay as
close
> >to the core as possible thereby using the headwind of the thermal.
>
> I'll defer to these sources:
>
> http://www.rc-soar.com/tech/thermals.htm
> Do thermals rotate?
> They do, but not predictably. Even dust devils don't have a
> preferred direction of rotation (see Stull, p.449). Thermals are
> too small and too short-lived to be affected by the earth's
> rotation (Coriolis force) or by the equator/pole thermal gradient.
> Their rotation is determined by local terrain. Rotational velocity
> in the core of a typical thermal is small compared to the vertical
> velocity.
>
>
> http://www.skynomad.com/articles/athermal10.html
> THERMALS
>
> Do Thermals Spin?
> by Peter Gray
>
> Of course, dust devils spin, so thermals probably do also, at
> least when they're close to the ground. For many years, I was
> convinced that dust devils rotated in random directions. However,
> based on a few flights in northern Washington State, where I kept
> more careful mental notes, I would guess that about 2/3 to 3/4 of
> the dust devils there spin counterclockwise, in agreement with the
> Coriolis Effect. Contrary to popular belief, the smaller-scale
> equivalent, water going down a toilet or drain, is essentially
> unaffected, and the rotation, if it is biased toward one spin
> direction, is the result of the geometry of each such basin (check
> it out for yourself!).
>
> The dust devils formed by thermals seem to be just large
> enough to be affected somewhat by the Coriolis Effect. If I have
> the choice, I usually opt for circling against the dust devil's
> rotation, most often to the right, in case this will produce a
> better climb rate by reducing my circling ground speed, thus bank
> angle and sinkrate.
>
> However, when I have reversed direction several times in one
> climb, I have rarely detected a significant advantage in one
> direction. What little angular momentum thermals start with, they
> seem to lose through drag in the surrounding air, and they
> probably start with little spin anyway. As with water going down a
> drain, very little spin momentum at a large radius can translate
> to a rapid spin when the radius shrinks to that of a dust devil.
>
> >I have never found a meteorological reason to circle in a certain
direction.
> >Most pilots circle lefthanded just because they were tought to do so in
the
> >vicinity of their home field. However in x-country flights there is
usually
> >no preference. In low situations the pilot usually circles in the
direction
> >he feels most comfortable. Sometimes the gliders feels better when
circling
> >to the right rather then to the left is my experience.
> >
> >Karel, NL
>
> Agreed. Circling direction is more often dictated by other gliders in
> the thermal than meteorological phenomena and physics.
>

[Charles Yeates]

Maybe it is simply an ergonomics thing --- I feel more comfortable
circling to the right in clutch situations simply because it is easier
to pull and adjust rather than push and adjust in right turns.

K.P. Termaat wrote:
> Thanks Larry for your interesting respons with your links to the articles.
>
> As a matter of fact most members of our National Team (my son is one of
> them) tend to circle to the right when low. They don't in effect know why.
> They say, and I agree, that it usually feels better in getting up again. So
> Peter Gray may be right in saying that most of the thermals have the
> tendency to rotate to the left in the northern hemisphere, especially when
> low when still having small diameters. Coriolis may be the cause then. Would
> be interesting to know the experience in the southern hemisphere.
>
> Talking about a drain and water brings me to the idea of telling that when
> low and looking for a thermal I always try to locate small ponds in dry
> areas. It looks to me that the water vapor rising from these ponds is an
> excellent means of starting a thermal. Water vapor is lighter then air, so
> it increases the boyancy of the air over the pond and off it goes. Starting
> at about ground level, circling to the right may then generally be the
> better option. Any experience with this Larry?
>
> Karel, NL
>
>
> "Larry Dighera" > schreef in bericht
> ...
>
>>On Fri, 26 Dec 2003 09:00:24 GMT, "K.P. Termaat" > wrote
>>in Message-Id: >:
>>
>>
>>>Larry wrote:
>>>
>>>
>>>>On an average day, a pilot will
>>>>spend a great deal of time attempting to "core" thermals. That
>>>>involves mentally visualizing the sailplane's position relative to the
>>>>thermal's vertical anticyclone column center, and guiding his
>>>>sailplane to circle as near to the center of it as he is able.
>>>
>>>Why do you write "anticyclone" Larry.
>>
>>That was my recollection from my soaring experience in the early '70s.
>>However, it appears that the word 'cyclone' may have more correct now
>>that I have done some research:
>>
>>The National Weather Service Glossary page here
>>http://www.wrh.noaa.gov/Phoenix/general/glossary/ has this to say:
>>
>> ANTICYCLONE - A region of high atmospheric pressure around which
>> winds move in a clockwise direction in the northern hemisphere.
>> Generally associated with warm and dry weather
>>
>> CYCLONE - An area of low pressure around which winds blow in a
>> counter-clockwise fashion in the northern hemisphere. Generally
>> associated with cool, wet and unsettled weather
>>
>>On the other hand ...
>>
>>
>>>This would mean that all gliders would circle righthanded to stay as
>>
> close
>
>>>to the core as possible thereby using the headwind of the thermal.
>>
>>I'll defer to these sources:
>>
>>http://www.rc-soar.com/tech/thermals.htm
>> Do thermals rotate?
>> They do, but not predictably. Even dust devils don't have a
>> preferred direction of rotation (see Stull, p.449). Thermals are
>> too small and too short-lived to be affected by the earth's
>> rotation (Coriolis force) or by the equator/pole thermal gradient.
>> Their rotation is determined by local terrain. Rotational velocity
>> in the core of a typical thermal is small compared to the vertical
>> velocity.
>>
>>
>>http://www.skynomad.com/articles/athermal10.html
>> THERMALS
>>
>> Do Thermals Spin?
>> by Peter Gray
>>
>> Of course, dust devils spin, so thermals probably do also, at
>> least when they're close to the ground. For many years, I was
>> convinced that dust devils rotated in random directions. However,
>> based on a few flights in northern Washington State, where I kept
>> more careful mental notes, I would guess that about 2/3 to 3/4 of
>> the dust devils there spin counterclockwise, in agreement with the
>> Coriolis Effect. Contrary to popular belief, the smaller-scale
>> equivalent, water going down a toilet or drain, is essentially
>> unaffected, and the rotation, if it is biased toward one spin
>> direction, is the result of the geometry of each such basin (check
>> it out for yourself!).
>>
>> The dust devils formed by thermals seem to be just large
>> enough to be affected somewhat by the Coriolis Effect. If I have
>> the choice, I usually opt for circling against the dust devil's
>> rotation, most often to the right, in case this will produce a
>> better climb rate by reducing my circling ground speed, thus bank
>> angle and sinkrate.
>>
>> However, when I have reversed direction several times in one
>> climb, I have rarely detected a significant advantage in one
>> direction. What little angular momentum thermals start with, they
>> seem to lose through drag in the surrounding air, and they
>> probably start with little spin anyway. As with water going down a
>> drain, very little spin momentum at a large radius can translate
>> to a rapid spin when the radius shrinks to that of a dust devil.
>>
>>
>>>I have never found a meteorological reason to circle in a certain
>>
> direction.
>
>>>Most pilots circle lefthanded just because they were tought to do so in
>>
> the
>
>>>vicinity of their home field. However in x-country flights there is
>>
> usually
>
>>>no preference. In low situations the pilot usually circles in the
>>
> direction
>
>>>he feels most comfortable. Sometimes the gliders feels better when
>>
> circling
>
>>>to the right rather then to the left is my experience.
>>>
>>>Karel, NL
>>
>>Agreed. Circling direction is more often dictated by other gliders in
>>the thermal than meteorological phenomena and physics.
>>
>
>
>

[Eric Greenwell]

Charles Yeates wrote:
> Maybe it is simply an ergonomics thing --- I feel more comfortable
> circling to the right in clutch situations simply because it is easier
> to pull and adjust rather than push and adjust in right turns.

For me, it's just habit: when I was learning to fly, we'd always get off
tow in a thermal; since we'd always depart tow with a turn to the right,
it was easiest to just continue turning right in the thermal.

For a few flights before a contest, I'll make an effort to fly at least
half my thermals to the left, so I'm ready for the left turns required
at US contests near the start and at turnpoints.

--
-----
change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

[Chris Rowland]

On Sat, 27 Dec 2003 10:06:17 -0800, Eric Greenwell
> wrote:

>Charles Yeates wrote:
>> Maybe it is simply an ergonomics thing --- I feel more comfortable
>> circling to the right in clutch situations simply because it is easier
>> to pull and adjust rather than push and adjust in right turns.
>
>For me, it's just habit: when I was learning to fly, we'd always get off
>tow in a thermal; since we'd always depart tow with a turn to the right,
>it was easiest to just continue turning right in the thermal.

Its my opinion that some thermals work better turning one way rather
than the other. If it doesn't seem to be working one way I'll reverse
the turn and try the other way. Sometimes it helps.

I get the impression there are days where the thermals are right
handed (or left handed). I've no idea if this is a coriolis thing
(although if somebiody wants to sponsor me to do a research project
involving a few months Gliding in OZ and NZ I wouldn't mind:-).

And some gliders, particularly much repaired club gliders, seem to
turn better one way rather than the other. It was a revelation when I
first flew something that was straight, it could turn equally well
both ways.

>
>For a few flights before a contest, I'll make an effort to fly at least
>half my thermals to the left, so I'm ready for the left turns required
>at US contests near the start and at turnpoints.

Yes, If I find I'm getting handed I'll do this, just because I always
want the choice.

Chris

[Gary Kemp]

Eric Greenwell > wrote in message >...
> Charles Yeates wrote:
> > Maybe it is simply an ergonomics thing --- I feel more comfortable
> > circling to the right in clutch situations simply because it is easier
> > to pull and adjust rather than push and adjust in right turns.
>
> For me, it's just habit: when I was learning to fly, we'd always get off
> tow in a thermal; since we'd always depart tow with a turn to the right,
> it was easiest to just continue turning right in the thermal.
>
> For a few flights before a contest, I'll make an effort to fly at least
> half my thermals to the left, so I'm ready for the left turns required
> at US contests near the start and at turnpoints.

I know a few competition pilots that are definitely "handed", I will
make it a point, when they are in my area, to turn in the direction,
they least prefer. Thanks, Eric

[Chris OCallaghan]

Two out of three thermals (very unscientific study on my part) favor a
right turn. This is the result of the internal circulation of the
thermal. This adds yet another variable to the complexity of
thermalling. I'll leave it up to you to figure out how lift strength,
horizontal and vertical lift profile, vorticity, and turn direction
affect climb performance. Suffice to say in this short note that they
do.

There was a discussion in Soaring (maybe 15 years ago) on this
subject, with a nay sayer concluding, if thermals had spiral
circulation, you could look up and see clouds rotating. Well, I spent
an hour on my back the following weekend and found that they did, and
favored clockwise 2 to 1.

I use the right hand rule. By pointing my thumb up (lift) and curling
my fingers (as in induction fields), I can determine with about 66%
probability the direction of rotation of the thermal as a whole.

A friend of mine (no longer an active glider pilot) used to keep a
careful watch on his yaw string as he entered thermals. He felt that
you could briefly see the direction of rotation and planned his
centering turn in the same direction as its initial deflection. A
talented pilot, I took careful note, though I admit I've defaulted to
always using a right turn unless there are clear indications that I
should turn left (noticeable yaw string deflection, birds, other
gliders, visible shear beneath cloudbase, a dramatic surge up of the
right wing, etc.)

As a side note, I've found that reversing direction is a useful
technique for climbing through a shear. This may be a result of
reverasal of rotation at the top of the inversion layer, but that's
just a guess.

OC

[Phoebus_810]

Eric Greenwell > wrote in message >...
> Charles Yeates wrote:
> > Maybe it is simply an ergonomics thing --- I feel more comfortable
> > circling to the right in clutch situations simply because it is easier
> > to pull and adjust rather than push and adjust in right turns.
>
> For me, it's just habit: when I was learning to fly, we'd always get off
> tow in a thermal; since we'd always depart tow with a turn to the right,
> it was easiest to just continue turning right in the thermal.
>
> For a few flights before a contest, I'll make an effort to fly at least
> half my thermals to the left, so I'm ready for the left turns required
> at US contests near the start and at turnpoints.

In Finland we consider it aerobatics to fly right turn in a termal.

[Eric Greenwell]

Chris OCallaghan wrote:
> Two out of three thermals (very unscientific study on my part) favor a
> right turn. This is the result of the internal circulation of the
> thermal. This adds yet another variable to the complexity of
> thermalling. I'll leave it up to you to figure out how lift strength,
> horizontal and vertical lift profile, vorticity, and turn direction
> affect climb performance. Suffice to say in this short note that they
> do.
>
> There was a discussion in Soaring (maybe 15 years ago) on this
> subject, with a nay sayer concluding, if thermals had spiral
> circulation, you could look up and see clouds rotating. Well, I spent
> an hour on my back the following weekend and found that they did, and
> favored clockwise 2 to 1.

Where do you fly? I've never seen clouds rotating, and I've been soaring
for 25 years. I have seen dust and other objects in thermals rotating
several thousand feet AGL.

--
-----
change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

[Casey Wilson]

>>
> >>Agreed. Circling direction is more often dictated by other gliders in
> >>the thermal than meteorological phenomena and physics.
> >>

The protocol I was taught was that unless you are first into the
thermal, you follow the left or right pattern of the gliders already there.
I've never had a preference of right or left. I was taught to turn into
whichever wingtip went up.
I was also taught that the most efficient technique, that is the
highest rate of altitude gain, is in a 45-degree bank turn hopefully
"coring" the thermal.
Up here in the Mojave Desert flying out of IYK, I've been in a couple of
10 Knot thermals but 5 to 6 is the most common. I can't ever remember any
kind of cyclonic rotation of any of them.
That said, I did once, inadvertantly fly into a dust-devil. I NEVER want
to do that again. If I had seen any dust indication that it was there I
would definitely have avoided it in the first place.

[Mark James Boyd]

Chris OCallaghan > wrote:
>
>There was a discussion in Soaring (maybe 15 years ago) on this
>subject, with a nay sayer concluding, if thermals had spiral
>circulation, you could look up and see clouds rotating. Well, I spent
>an hour on my back the following weekend and found that they did, and
>favored clockwise 2 to 1.
>
>I use the right hand rule. By pointing my thumb up (lift) and curling
>my fingers (as in induction fields), I can determine with about 66%
>probability the direction of rotation of the thermal as a whole.
>

Jack Glendening, among others, have discounted the idea that
something as small as a thermal rotates in a particular
direction due to any Coriolis forces.

However, I carefully examined my .igc files for
extended climbs in thermals, and found that my climb
looks like a "corkscrew." I'm not talking about
the path of the glider, but the center of the
thermal at the various altitudes. As far as I can tell,
this had nothing to do with the direction the
wind rotates in the thermal, but is simply the thermal
itself not going "up" straight (or even slanted evenly
due to wind). So perhaps coriolis affects the thermal, or
perhaps it affects the way the GPS readings are determined.
In either case, I'll need to go back and look again at
these traces.

Has anyone else noticed that the core of the thermal
they are climbing in seems to rise like a corkscrew
with altitude? Does anyone have a hypothesis about if
it is an effect on the thermal, or on the GPS?

[Andreas Maurer]

On Mon, 29 Dec 2003 02:02:43 GMT, "Casey Wilson" >
wrote:

> That said, I did once, inadvertantly fly into a dust-devil. I NEVER want
>to do that again. If I had seen any dust indication that it was there I
>would definitely have avoided it in the first place.

Could you tell more please?
This is one of the things that I've been thinking about for a long
time.


Bye
Andreas

[Eric Greenwell]

Casey Wilson wrote:

> That said, I did once, inadvertantly fly into a dust-devil. I NEVER want
> to do that again. If I had seen any dust indication that it was there I
> would definitely have avoided it in the first place.

No dust, no dust devil! But, of course, the thermal can still be there.
Coming into one low can be dangerous, but up here in eastern Washington
State, we use them frequently, especially on blue days. They are usually
quite tame. Only the biggest are potentially dangerous, and then only
when "near" the ground (say, less then 1500 feet AGL).

What you are flying makes a difference, too: a 1-26 is going to be
tossed around a lot more than an ASW 20 with ballast. Flying faster than
the normal thermalling speed helps quite a bit if the thermal is rough.


--
-----
change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

[F.L. Whiteley]

"Mark James Boyd" > wrote in message
news:3fefacf7$1@darkstar...
> Chris OCallaghan > wrote:
> >
> >There was a discussion in Soaring (maybe 15 years ago) on this
> >subject, with a nay sayer concluding, if thermals had spiral
> >circulation, you could look up and see clouds rotating. Well, I spent
> >an hour on my back the following weekend and found that they did, and
> >favored clockwise 2 to 1.
> >
> >I use the right hand rule. By pointing my thumb up (lift) and curling
> >my fingers (as in induction fields), I can determine with about 66%
> >probability the direction of rotation of the thermal as a whole.
> >
>
> Jack Glendening, among others, have discounted the idea that
> something as small as a thermal rotates in a particular
> direction due to any Coriolis forces.
>
> However, I carefully examined my .igc files for
> extended climbs in thermals, and found that my climb
> looks like a "corkscrew." I'm not talking about
> the path of the glider, but the center of the
> thermal at the various altitudes. As far as I can tell,
> this had nothing to do with the direction the
> wind rotates in the thermal, but is simply the thermal
> itself not going "up" straight (or even slanted evenly
> due to wind). So perhaps coriolis affects the thermal, or
> perhaps it affects the way the GPS readings are determined.
> In either case, I'll need to go back and look again at
> these traces.
>
> Has anyone else noticed that the core of the thermal
> they are climbing in seems to rise like a corkscrew
> with altitude? Does anyone have a hypothesis about if
> it is an effect on the thermal, or on the GPS?
>
>
According to some published research by Wayne Angevine,
http://cires.colorado.edu/ and NOAA, and the results of analysing thermic
activity with lidar and IR methods in the mid-1990's, you might do better to
think of thermals as plume-like. You may transit one or more plumes in a
thermaling turn and plumes may merge into larger plumes while rising.
Merging would seem to account for those lateral shifts, strengthening gusts,
and some rotational effects. Transiting multiple smaller plumes might
explain why some thermals appear to have two or more 'cores' or are hard to
center.

Wayne spoke at our club on this a few years back. His article was published
in one of the model magazines and research presented at the CIRES
gatherings. One of his grad student researchers was a collegiate member of
our club during this period. Wayne's specialty is atmospheric boundary
layer physics.

Frank Whiteley

[F.L. Whiteley]

http://www.rc-soar.com/tech/thermals.htm

Early version of Wayne's work. He presented us with quite a detailed
display of a cubic mile of atmospheric activity.

Frank

"F.L. Whiteley" > wrote in message
...
>
> "Mark James Boyd" > wrote in message
> news:3fefacf7$1@darkstar...
> > Chris OCallaghan > wrote:
> > >
> > >There was a discussion in Soaring (maybe 15 years ago) on this
> > >subject, with a nay sayer concluding, if thermals had spiral
> > >circulation, you could look up and see clouds rotating. Well, I spent
> > >an hour on my back the following weekend and found that they did, and
> > >favored clockwise 2 to 1.
> > >
> > >I use the right hand rule. By pointing my thumb up (lift) and curling
> > >my fingers (as in induction fields), I can determine with about 66%
> > >probability the direction of rotation of the thermal as a whole.
> > >
> >
> > Jack Glendening, among others, have discounted the idea that
> > something as small as a thermal rotates in a particular
> > direction due to any Coriolis forces.
> >
> > However, I carefully examined my .igc files for
> > extended climbs in thermals, and found that my climb
> > looks like a "corkscrew." I'm not talking about
> > the path of the glider, but the center of the
> > thermal at the various altitudes. As far as I can tell,
> > this had nothing to do with the direction the
> > wind rotates in the thermal, but is simply the thermal
> > itself not going "up" straight (or even slanted evenly
> > due to wind). So perhaps coriolis affects the thermal, or
> > perhaps it affects the way the GPS readings are determined.
> > In either case, I'll need to go back and look again at
> > these traces.
> >
> > Has anyone else noticed that the core of the thermal
> > they are climbing in seems to rise like a corkscrew
> > with altitude? Does anyone have a hypothesis about if
> > it is an effect on the thermal, or on the GPS?
> >
> >
> According to some published research by Wayne Angevine,
> http://cires.colorado.edu/ and NOAA, and the results of analysing thermic
> activity with lidar and IR methods in the mid-1990's, you might do better
to
> think of thermals as plume-like. You may transit one or more plumes in a
> thermaling turn and plumes may merge into larger plumes while rising.
> Merging would seem to account for those lateral shifts, strengthening
gusts,
> and some rotational effects. Transiting multiple smaller plumes might
> explain why some thermals appear to have two or more 'cores' or are hard
to
> center.
>
> Wayne spoke at our club on this a few years back. His article was
published
> in one of the model magazines and research presented at the CIRES
> gatherings. One of his grad student researchers was a collegiate member
of
> our club during this period. Wayne's specialty is atmospheric boundary
> layer physics.
>
> Frank Whiteley
>
>

[Mike Rapoport]

"K.P. Termaat" > wrote in message
...
> Thanks Larry for your interesting respons with your links to the articles.
>
>
> Talking about a drain and water brings me to the idea of telling that when
> low and looking for a thermal I always try to locate small ponds in dry
> areas. It looks to me that the water vapor rising from these ponds is an
> excellent means of starting a thermal. Water vapor is lighter then air, so
> it increases the boyancy of the air over the pond and off it goes.
Starting
> at about ground level, circling to the right may then generally be the
> better option. Any experience with this Larry?
>
> Karel, NL
>
You will find less lift over water of any kind, even if it is contained in
vegetation. The best lift is always over the highest, dryest, darkest
surface around. The water vapor idea is...well...it is hard to find a place
to start...but it won't work

Mike
MU-2

[Fantsu]

"Phoebus_810" > wrote in message
m...
> Eric Greenwell > wrote in message
>...
> > Charles Yeates wrote:
> > > Maybe it is simply an ergonomics thing --- I feel more comfortable
> > > circling to the right in clutch situations simply because it is easier
> > > to pull and adjust rather than push and adjust in right turns.
> >
> > For me, it's just habit: when I was learning to fly, we'd always get off
> > tow in a thermal; since we'd always depart tow with a turn to the right,
> > it was easiest to just continue turning right in the thermal.
> >
> > For a few flights before a contest, I'll make an effort to fly at least
> > half my thermals to the left, so I'm ready for the left turns required
> > at US contests near the start and at turnpoints.
>
> In Finland we consider it aerobatics to fly right turn in a termal.

Eh..then we don't fly in the same Finland ;)

Most competition pilots at least can fly them bothways, though near the
airfield in comps turning left is obligatory (and it's also the way to get
away from the tug)

h

[K.P. Termaat]

My experience is that it works, especially on days with very low humidity,
but no boomers and only low.
I'am talking about small shallow ponds in dry area's especially when the
ponds are surrounded by sandy grounds with higher vegetation like trees. The
buoyancy impuls from the evaporated water is apparently just good enough to
start the thermal which then sucks air from its heated up vicinity.
Has saved me many times when I was still flying my Pik20D or more recently
my DG800S.

Karel, NL
V-2cxT





"Mike Rapoport" > schreef in bericht
ink.net...
>
> "K.P. Termaat" > wrote in message
> ...
> > Thanks Larry for your interesting respons with your links to the
articles.
> >
> >
> > Talking about a drain and water brings me to the idea of telling that
when
> > low and looking for a thermal I always try to locate small ponds in dry
> > areas. It looks to me that the water vapor rising from these ponds is an
> > excellent means of starting a thermal. Water vapor is lighter then air,
so
> > it increases the boyancy of the air over the pond and off it goes.
> Starting
> > at about ground level, circling to the right may then generally be the
> > better option. Any experience with this Larry?
> >
> > Karel, NL
> >
> You will find less lift over water of any kind, even if it is contained in
> vegetation. The best lift is always over the highest, dryest, darkest
> surface around. The water vapor idea is...well...it is hard to find a
place
> to start...but it won't work
>
> Mike
> MU-2
>
>

[Phoebus_810]

"Fantsu" > wrote in message >...
> "Phoebus_810" > wrote in message
> m...
> > Eric Greenwell > wrote in message
> >...
> > > Charles Yeates wrote:
> > > > Maybe it is simply an ergonomics thing --- I feel more comfortable
> > > > circling to the right in clutch situations simply because it is easier
> > > > to pull and adjust rather than push and adjust in right turns.
> > >
> > > For me, it's just habit: when I was learning to fly, we'd always get off
> > > tow in a thermal; since we'd always depart tow with a turn to the right,
> > > it was easiest to just continue turning right in the thermal.
> > >
> > > For a few flights before a contest, I'll make an effort to fly at least
> > > half my thermals to the left, so I'm ready for the left turns required
> > > at US contests near the start and at turnpoints.
> >
> > In Finland we consider it aerobatics to fly right turn in a termal.
>
> Eh..then we don't fly in the same Finland ;)
>
> Most competition pilots at least can fly them bothways, though near the
> airfield in comps turning left is obligatory (and it's also the way to get
> away from the tug)
>
> h

It was a joke!!

[William W. Plummer]

I didn't receive any direction about which way to circle. Common sense
would rule out going against the traffic in an established pattern.

If there is a "standard" I wonder if it is related to the default for holds
(right) in airplanes. I used to wonder why the default for holds is to the
right but the default for VFR patterns at airports is left. --Bill



"Casey Wilson" > wrote in message
...
> >>
> > >>Agreed. Circling direction is more often dictated by other gliders in
> > >>the thermal than meteorological phenomena and physics.
> > >>
>
> The protocol I was taught was that unless you are first into the
> thermal, you follow the left or right pattern of the gliders already
there.
> I've never had a preference of right or left. I was taught to turn
into
> whichever wingtip went up.
> I was also taught that the most efficient technique, that is the
> highest rate of altitude gain, is in a 45-degree bank turn hopefully
> "coring" the thermal.
> Up here in the Mojave Desert flying out of IYK, I've been in a couple
of
> 10 Knot thermals but 5 to 6 is the most common. I can't ever remember any
> kind of cyclonic rotation of any of them.
> That said, I did once, inadvertantly fly into a dust-devil. I NEVER
want
> to do that again. If I had seen any dust indication that it was there I
> would definitely have avoided it in the first place.
>
>

[K.P. Termaat]

The "standard" is to circle to the left. Usually this is a must near glider
airfields, especially with a competition going on. This is for safety
reasons.
While on x-country and joining other gliders, the circle direction is given
by these other gliders in the same thermal.
When alone and hitting a thermal, the direction to turn into is where you
suspect the thermal to be. Some experience is required for that. It always
makes sense to circle towards the direction of rotation of the thermal if
there is any rotation. Some pilots have the idea that 2/3 of the thermals
rotate counter clockwise looking from below. I have the idea that this may
be true, but only at low altitudes. That's why quite a lot of pilots, and I
am one of them, circle to the right when low. Some other pilots think that
there is no rotation at all, except in very narrow dust devils which are
unsuitable to fly in.

Karel, NL
"William W. Plummer" > schreef in bericht
news:gCWHb.163786$8y1.490686@attbi_s52...
> I didn't receive any direction about which way to circle. Common sense
> would rule out going against the traffic in an established pattern.
>
> If there is a "standard" I wonder if it is related to the default for
holds
> (right) in airplanes. I used to wonder why the default for holds is to
the
> right but the default for VFR patterns at airports is left. --Bill
>
>
>
> "Casey Wilson" > wrote in message
> ...
> > >>
> > > >>Agreed. Circling direction is more often dictated by other gliders
in
> > > >>the thermal than meteorological phenomena and physics.
> > > >>
> >
> > The protocol I was taught was that unless you are first into the
> > thermal, you follow the left or right pattern of the gliders already
> there.
> > I've never had a preference of right or left. I was taught to turn
> into
> > whichever wingtip went up.
> > I was also taught that the most efficient technique, that is the
> > highest rate of altitude gain, is in a 45-degree bank turn hopefully
> > "coring" the thermal.
> > Up here in the Mojave Desert flying out of IYK, I've been in a
couple
> of
> > 10 Knot thermals but 5 to 6 is the most common. I can't ever remember
any
> > kind of cyclonic rotation of any of them.
> > That said, I did once, inadvertantly fly into a dust-devil. I NEVER
> want
> > to do that again. If I had seen any dust indication that it was there I
> > would definitely have avoided it in the first place.
> >
> >
>
>

[Kirk Stant]

"K.P. Termaat" > wrote in message >...
> My experience is that it works, especially on days with very low humidity,
> but no boomers and only low.
>
>
> "Mike Rapoport" > schreef in bericht
> ink.net...
> >
> > You will find less lift over water of any kind, even if it is contained in
> > vegetation. The best lift is always over the highest, dryest, darkest
> > surface around. The water vapor idea is...well...it is hard to find a
> place
> > to start...but it won't work
> >
> > Mike
> > MU-2
> >

Have to disagree with you, Mike - out here in Arizona, in the desert
areas that are not irrigated, we often find good lift directly over
small cattle "tanks" - small shallow ponds that are scattered around.
A lot of us have noticed this and compared notes, and it works; if too
low to get to high, dark ground, I'll head for the nearest pond and it
will usually turn up a nice thermal. We think it may be due to the
fact that the ponds are in a natural low spot, and coupled with the
little bit of moisture, could be the necessary trigger for a thermal.

Now obviously, large irrigated farm fields or river basins are death
to thermals - but a local lake (reservoir) seems to have little effect
on thermal activity - could it be all the drunk boaters?

What's the old saying about never saying never?

Kirk
LS6-b

[Mike Rapoport]

The moisture doesn't really help lift until the air is fuly saturated and
starts condensing releasing heat. Also, the air above the water is cooled
by evaporation and is cooler than the surrounding air. I will never say
never and I don't dispute your or others experience, but the explanation
doesn't make sense to me.

Mike
MU-2


"Kirk Stant" > wrote in message
om...
> "K.P. Termaat" > wrote in message
>...
> > My experience is that it works, especially on days with very low
humidity,
> > but no boomers and only low.
> >
> >
> > "Mike Rapoport" > schreef in bericht
> > ink.net...
> > >
> > > You will find less lift over water of any kind, even if it is
contained in
> > > vegetation. The best lift is always over the highest, dryest, darkest
> > > surface around. The water vapor idea is...well...it is hard to find a
> > place
> > > to start...but it won't work
> > >
> > > Mike
> > > MU-2
> > >
>
> Have to disagree with you, Mike - out here in Arizona, in the desert
> areas that are not irrigated, we often find good lift directly over
> small cattle "tanks" - small shallow ponds that are scattered around.
> A lot of us have noticed this and compared notes, and it works; if too
> low to get to high, dark ground, I'll head for the nearest pond and it
> will usually turn up a nice thermal. We think it may be due to the
> fact that the ponds are in a natural low spot, and coupled with the
> little bit of moisture, could be the necessary trigger for a thermal.
>
> Now obviously, large irrigated farm fields or river basins are death
> to thermals - but a local lake (reservoir) seems to have little effect
> on thermal activity - could it be all the drunk boaters?
>
> What's the old saying about never saying never?
>
> Kirk
> LS6-b

[Andy Durbin]

"Mike Rapoport" > wrote in message et>...

> >
> You will find less lift over water of any kind, even if it is contained in
> vegetation. The best lift is always over the highest, dryest, darkest
> surface around. The water vapor idea is...well...it is hard to find a place
> to start...but it won't work
>
> Mike
> MU-2


I hope you come and fly with us in Arizona some time. After a few
miles of cross country in the blue you may come to appreciate the
thermals triggered by the small ponds known as cattle tanks.

Been using them for over 15 years and no theororetical analysis will
convince me they dont work.


Andy (GY)

[Peter Creswick]

Mike Rapoport wrote:
>
> The moisture doesn't really help lift until the air is fuly saturated and
> starts condensing releasing heat. Also, the air above the water is cooled
> by evaporation and is cooler than the surrounding air. I will never say
> never and I don't dispute your or others experience, but the explanation
> doesn't make sense to me.
>
> Mike
> MU-2
>
> "Kirk Stant" > wrote in message
> om...
> > "K.P. Termaat" > wrote in message
> >...
> > > My experience is that it works, especially on days with very low
> humidity,
> > > but no boomers and only low.
> > >
> > >
> > > "Mike Rapoport" > schreef in bericht
> > > ink.net...
> > > >
> > > > You will find less lift over water of any kind, even if it is
> contained in
> > > > vegetation. The best lift is always over the highest, dryest, darkest
> > > > surface around. The water vapor idea is...well...it is hard to find a
> > > place
> > > > to start...but it won't work
> > > >
> > > > Mike
> > > > MU-2
> > > >
> >
> > Have to disagree with you, Mike - out here in Arizona, in the desert
> > areas that are not irrigated, we often find good lift directly over
> > small cattle "tanks" - small shallow ponds that are scattered around.
> > A lot of us have noticed this and compared notes, and it works; if too
> > low to get to high, dark ground, I'll head for the nearest pond and it
> > will usually turn up a nice thermal. We think it may be due to the
> > fact that the ponds are in a natural low spot, and coupled with the
> > little bit of moisture, could be the necessary trigger for a thermal.
> >
> > Now obviously, large irrigated farm fields or river basins are death
> > to thermals - but a local lake (reservoir) seems to have little effect
> > on thermal activity - could it be all the drunk boaters?
> >
> > What's the old saying about never saying never?
> >
> > Kirk
> > LS6-b

Have seen similar effects over the small dams on farms here too. My idea is that the air over the
water cools by evaporating water out of the pond. In so doing it looses more heat and hence
contracts more (gets denser) than it gains buoyancy by water vapour increase, ie, it gets both
colder and denser overall than the surrounding surface air. As the dense pool of air becomes
greater, it spreads out, ie, sort of collapses on itself, and pushes out over the edges of the pond
/ dam, particularly down slope over the dam wall, creating a miniature equivalent of a valley wind
in the creek or down the slope, thus acting as a wedge trigger to lift the warm dry air off the
ground.

[Mike Rapoport]

I am not disputing the facts, I am disputing the explanation.

Mike
MU-2


"Andy Durbin" > wrote in message
om...
> "Mike Rapoport" > wrote in message
et>...
>
> > >
> > You will find less lift over water of any kind, even if it is contained
in
> > vegetation. The best lift is always over the highest, dryest, darkest
> > surface around. The water vapor idea is...well...it is hard to find a
place
> > to start...but it won't work
> >
> > Mike
> > MU-2
>
>
> I hope you come and fly with us in Arizona some time. After a few
> miles of cross country in the blue you may come to appreciate the
> thermals triggered by the small ponds known as cattle tanks.
>
> Been using them for over 15 years and no theororetical analysis will
> convince me they dont work.
>
>
> Andy (GY)

[Peter Creswick]

Mike Rapoport wrote:
>
> "K.P. Termaat" > wrote in message
> ...
> > Thanks Larry for your interesting respons with your links to the articles.
> >
> >
> > Talking about a drain and water brings me to the idea of telling that when
> > low and looking for a thermal I always try to locate small ponds in dry
> > areas. It looks to me that the water vapor rising from these ponds is an
> > excellent means of starting a thermal. Water vapor is lighter then air, so
> > it increases the boyancy of the air over the pond and off it goes.
> Starting
> > at about ground level, circling to the right may then generally be the
> > better option. Any experience with this Larry?
> >
> > Karel, NL
> >
> You will find less lift over water of any kind, even if it is contained in
> vegetation. The best lift is always over the highest, dryest, darkest
> surface around. The water vapor idea is...well...it is hard to find a place
> to start...but it won't work
>
> Mike
> MU-2

As I just posted in the "water vapour" thread, I think you have it all back the front.
Have seen similar effects over the small dams on farms here too. My idea is that the air over the
water cools by evaporating water out of the pond. In so doing it looses more heat and hence
contracts more (gets denser) than it gains buoyancy by water vapour increase, ie, it gets both
colder and denser overall than the surrounding surface air. As the dense pool of air becomes
greater, it spreads out, ie, sort of collapses on itself, and pushes out over the edges of the pond
/ dam, particularly down slope over the dam wall, creating a miniature equivalent of a valley wind
in the creek or down the slope, thus acting as a wedge trigger to lift the warm dry air off the
ground.

[Chris OCallaghan]

Frank, your description of Wayne's model is closest to my own
observation. There is perceptible vorticity in cumulus clouds, both on
a macro and micro scale (within the limits of the thermal), that is,
you can perceive a slow turning of the cloud as a whole, while
numerous smaller vortices are encompassed within it. I will read
Wayne's work with interest. To those that ascribe to the theory that
there is no rotation of the thermal, I would begin with the prompt to
"look up." Best if you can take 20 to 30 minutes and watch a thermal
throughout its life.

[Chris OCallaghan]

Mark,

Are you referring to a sinuousness or serpentine bending in the
vertical path of the core? That is, bending like some narrow cored
tornados?

[JJ Sinclair]

>
>I am not disputing the facts, I am disputing the explanation.
>

OK, Its time for my Minden pond theory. We have a small pond about 5 miles east
of the airport that consistently produces thermals. JJ's explanation; Heated
air that is slowely moving over the ground by the wind, suddenly comes to the
cooler pond edge. This cool edge of the pond acts as a trigger that forces the
heated air to break loose and start rising and that's why thermals can be found
at the edge of small ponds.
JJ Sinclair

[Larry Dighera]

On Mon, 29 Dec 2003 14:11:24 GMT, "William W. Plummer"
> wrote in Message-Id:
<gCWHb.163786$8y1.490686@attbi_s52>:

>I used to wonder why the default for holds is to the
>right but the default for VFR patterns at airports is left. --Bill

[Brian Case]

That is how I found my best thermal ever. Flying my HP16T at a Region
8 contest in Eric's stomping grounds. It was about 2 hours into a 3
Hour post task and front started moving into the area. I decided that
getting home might score better than trying to make the minimum time
and landing out so I headed back to the airport under the cloud deck.
The Glide was totally smooth and I arrived back at the airport at
about 1500 AGL. (~3000MSL) on the far side of the airport the sun was
shining and I could see two large dust devils up on the hill from the
airport. As I approached closer dust devil I could see cheat grass,
tumble weeds and garbage bags floating around in it. I put my left
wing into the dust devil and turned hard left. My 10kt vario pegged. I
switched the scale to 20kts and it pegged again. It would occasionall
drop to as low as 16kts. I went from 3000MSL to 11000MSL in about 4
minutes averaging right at 20kts. I had to stop the climb due to cloud
bases. From there I did a final glide out to a turnpoint and back to
the airport to finish within a few minutes of the 3 hour minimum.
Looking out on the wing I could see cheat grass draped all along the
leading edge of the wing. I am sure it didn't due anything good to my
glide ratio. As I rolled to a stop at the airport all the Cheat grass
dropped off the wing onto the tarmac, which made for a great story
"There I was, so low that..."

Brian
HP16T



Eric Greenwell > wrote in message >...
> Casey Wilson wrote:
>
> > That said, I did once, inadvertantly fly into a dust-devil. I NEVER want
> > to do that again. If I had seen any dust indication that it was there I
> > would definitely have avoided it in the first place.
>
> No dust, no dust devil! But, of course, the thermal can still be there.
> Coming into one low can be dangerous, but up here in eastern Washington
> State, we use them frequently, especially on blue days. They are usually
> quite tame. Only the biggest are potentially dangerous, and then only
> when "near" the ground (say, less then 1500 feet AGL).
>
> What you are flying makes a difference, too: a 1-26 is going to be
> tossed around a lot more than an ASW 20 with ballast. Flying faster than
> the normal thermalling speed helps quite a bit if the thermal is rough.

[Brian Case]

On the other hand I can't tell you how many times(numerous) I
intercepted a dust devil at 1000-1500 AGL and climbed out at less then
1kt or even did not climb. Most times however I get 3-6 kts out of
them. I have see dust devils go to 7-8000 feet up. Hate to tell the
one gentleman this that wouldn't fly into a dust devil, but if he flys
using thermals he is just flying into dustless dust devils, As far as
I can tell the only difference is if it is lifting air over an area
were it can pick dust up or not.

Brian
HP16T



Eric Greenwell > wrote in message >...
> Casey Wilson wrote:
>
> > That said, I did once, inadvertantly fly into a dust-devil. I NEVER want
> > to do that again. If I had seen any dust indication that it was there I
> > would definitely have avoided it in the first place.
>
> No dust, no dust devil! But, of course, the thermal can still be there.
> Coming into one low can be dangerous, but up here in eastern Washington
> State, we use them frequently, especially on blue days. They are usually
> quite tame. Only the biggest are potentially dangerous, and then only
> when "near" the ground (say, less then 1500 feet AGL).
>
> What you are flying makes a difference, too: a 1-26 is going to be
> tossed around a lot more than an ASW 20 with ballast. Flying faster than
> the normal thermalling speed helps quite a bit if the thermal is rough.

[F.L. Whiteley]

"Chris OCallaghan" > wrote in message
om...
> Frank, your description of Wayne's model is closest to my own
> observation. There is perceptible vorticity in cumulus clouds, both on
> a macro and micro scale (within the limits of the thermal), that is,
> you can perceive a slow turning of the cloud as a whole, while
> numerous smaller vortices are encompassed within it. I will read
> Wayne's work with interest. To those that ascribe to the theory that
> there is no rotation of the thermal, I would begin with the prompt to
> "look up." Best if you can take 20 to 30 minutes and watch a thermal
> throughout its life.

There's a later article in one of the modeling magazines complete with the
graphics (unfortunately I've don't recall these being reproduced on the web)
and more analysis. We'd asked to see if it could be re-published in
soaring, but apparently the those rights weren't available. Wayne is an RC
enthusiast AFAIK, the thermal modeling he showed us at the club talk
indicated plumes at higher altitudes than mentioned in the link. I believe
the CIRES team returned to the same cubic mile of air for 4-5 years and I
think it was in Illinois or perhaps Indiana (have to dig back), so he
modeling of thermals deals with some specific topography and weather
patterns. Funding was always an issue. He did say that the most reliable
thermal finder would need to 'see' detritous in the air.

Frank

[Chris OCallaghan]

Eric,

Look up!

I know, it's pretty impolite of me. I assure you it is nothing
personal against you in particular. Many, many pilots share your
opinion. I guess I find it very frustrating to have to explain the
obvious. Or. more to the point, to have to argue for the sake of the
obvious. My preference would be to move to the next level, to discuss
what the real effects of vorticity are on soaring flight. Instead, we
wallow about trying to apply models to decide the truth about
something easily observed.

Granted, our attention spans are short, but spending a half hour on
your back watching the development of a cumulus cloud can be very
instructive. But you needn't even do this if you are observant during
your climbs. You will note that around the edges of the cloud you'll
occasionally see vortices forming when the light hits them right. Or
if there is clag below cloud base, it will often have a discernable
rotation. When observed, they often herald localized cores that can be
exploited. More than once I've noted vorticity on the edege of a large
cloud, shifted my circle to it, and been rewarded with much improved
lift.

Spending some time on your back, you will observe that not only are
there localized vortices, but that the entire system slowly rotates.
Sometimes it's easier to see this than at others. However, patience is
required since the rotation is very slow, but nontheless discernable.
Best to start with a wisp and watch its full development. Choose one
as close to directly overhead as possible. The closer your view to
directly beneath, the more obvious the cloud rotation becomes.

I was hoping that the accelerated cloudscapes in Going the Distance
would make this obvious, but most of the clouds are shot at very low
angles. This makes discering the rotation more difficult as it is
overpowered by the vertical development and dissipation of the cloud.
By viewing from directly beneath, you won't be distracted by this.

Perhaps someone with the appropriate video recording capabilities
could do some time lapse photography and offer it up for review.

[Chris OCallaghan]

Most people can't see a minute hand moving, but it moves nonetheless.
It's a matter of patience. The problem with observing cloud rotation
is that the cloud is constantly changing shape, therefore you cannot
time lapse the same way you can when observing vertical development
(or determining that a minute hand moves). As noted in my impolite
post, you need to spend a half hour on your back. Then we can move on
to discussing whether there's any real advantage to be had.

[Andrew Sarangan]

(Kirk Stant) wrote in message >...
> "K.P. Termaat" > wrote in message >...
> > My experience is that it works, especially on days with very low humidity,
> > but no boomers and only low.
> >
> >
> > "Mike Rapoport" > schreef in bericht
> > ink.net...
> > >
> > > You will find less lift over water of any kind, even if it is contained in
> > > vegetation. The best lift is always over the highest, dryest, darkest
> > > surface around. The water vapor idea is...well...it is hard to find a
> place
> > > to start...but it won't work
> > >
> > > Mike
> > > MU-2
> > >
>
> Have to disagree with you, Mike - out here in Arizona, in the desert
> areas that are not irrigated, we often find good lift directly over
> small cattle "tanks" - small shallow ponds that are scattered around.
> A lot of us have noticed this and compared notes, and it works; if too
> low to get to high, dark ground, I'll head for the nearest pond and it
> will usually turn up a nice thermal. We think it may be due to the
> fact that the ponds are in a natural low spot, and coupled with the
> little bit of moisture, could be the necessary trigger for a thermal.
>
> Now obviously, large irrigated farm fields or river basins are death
> to thermals - but a local lake (reservoir) seems to have little effect
> on thermal activity - could it be all the drunk boaters?
>
> What's the old saying about never saying never?
>
> Kirk
> LS6-b


The adiabatic rate of moist air is about 1.5C/1000ft. For dry air it
is 3C/1000ft. Therefore, assuming that the water temperature is the
same as the surrounding ground (which would be true if the water is
shallow), I could see how one will experience greater lift above
water.

[Mike Rapoport]

"Andrew Sarangan" > wrote in message
om...
> (Kirk Stant) wrote in message
>...
> > "K.P. Termaat" > wrote in message
>...
> > > My experience is that it works, especially on days with very low
humidity,
> > > but no boomers and only low.
> > >
> > >
> > > "Mike Rapoport" > schreef in bericht
> > > ink.net...
> > > >
> > > > You will find less lift over water of any kind, even if it is
contained in
> > > > vegetation. The best lift is always over the highest, dryest,
darkest
> > > > surface around. The water vapor idea is...well...it is hard to find
a
> > place
> > > > to start...but it won't work
> > > >
> > > > Mike
> > > > MU-2
> > > >
> >
> > Have to disagree with you, Mike - out here in Arizona, in the desert
> > areas that are not irrigated, we often find good lift directly over
> > small cattle "tanks" - small shallow ponds that are scattered around.
> > A lot of us have noticed this and compared notes, and it works; if too
> > low to get to high, dark ground, I'll head for the nearest pond and it
> > will usually turn up a nice thermal. We think it may be due to the
> > fact that the ponds are in a natural low spot, and coupled with the
> > little bit of moisture, could be the necessary trigger for a thermal.
> >
> > Now obviously, large irrigated farm fields or river basins are death
> > to thermals - but a local lake (reservoir) seems to have little effect
> > on thermal activity - could it be all the drunk boaters?
> >
> > What's the old saying about never saying never?
> >
> > Kirk
> > LS6-b
>
>
> The adiabatic rate of moist air is about 1.5C/1000ft. For dry air it
> is 3C/1000ft. Therefore, assuming that the water temperature is the
> same as the surrounding ground (which would be true if the water is
> shallow), I could see how one will experience greater lift above
> water.

Not for unsaturated air it isn't. The moist rate only applies to saturated
air (ie in clouds)

Mike
MU-2

[Eric Greenwell]

Chris OCallaghan wrote:
>
> Granted, our attention spans are short, but spending a half hour on
> your back watching the development of a cumulus cloud can be very
> instructive.

We've been talking about dust devils, which rotate as fast as 20 times
minute, but you seem to be talking about rotations slower than 10 times
per hour. No wonder I don't notice rotations like that while flying.

> But you needn't even do this if you are observant during
> your climbs. You will note that around the edges of the cloud you'll
> occasionally see vortices forming when the light hits them right.

I do see these generally small clumps of rotations in the vertical
plane, near cloud edges. This isn't what I would call "clouds rotating"
in this context (which direction to circle), by which I mean a
substantial amount of the cloud going around horizontally like a dust
devil does. I have seen roll clouds rotating rapidly, but that isn't
relevant to dust devils.

> Or
> if there is clag below cloud base, it will often have a discernable
> rotation. When observed, they often herald localized cores that can be
> exploited.

I have never observed this, though I always head for "tendrils" or
clumps of cloud forming below the nominal cloud base, as the lift is
usually much better. The movement, if present, has been invariably
upwards. What speed are the cores rotating at? I'm sure I'd notice if it
was anything like dust devil speed.

> More than once I've noted vorticity on the edege of a large
> cloud, shifted my circle to it, and been rewarded with much improved
> lift.

Me too, but it's not been in the horizontal plane.

> Spending some time on your back, you will observe that not only are
> there localized vortices, but that the entire system slowly rotates.
> Sometimes it's easier to see this than at others. However, patience is
> required since the rotation is very slow, but nontheless discernable.

I haven't looked for rotation this slow, and while it's interesting, it
doesn't seem relevant to the choice of circling direction.

> Best to start with a wisp and watch its full development. Choose one
> as close to directly overhead as possible. The closer your view to
> directly beneath, the more obvious the cloud rotation becomes.

I'll try this next time I'm stuck on the ground under cumulus clouds.

[Casey Wilson]

"Brian Case" > wrote in message
om...
> On the other hand I can't tell you how many times(numerous) I
> intercepted a dust devil at 1000-1500 AGL and climbed out at less then
> 1kt or even did not climb. Most times however I get 3-6 kts out of
> them. I have see dust devils go to 7-8000 feet up. Hate to tell the
> one gentleman this that wouldn't fly into a dust devil, but if he flys
> using thermals he is just flying into dustless dust devils, As far as
> I can tell the only difference is if it is lifting air over an area
> were it can pick dust up or not.

We are apparently sharing different definitions of "dust devils." The
ones that rage across the part of the country I fly mostly do not fit the
structure of a thermal.
Thermals, at least where I am, are rising volumes of air created by
differential temperatures on the surface. One of our best local thermal
engines is the black paved surface of Runway 10/28 at IYK. The equipment
parking lot for the highway maintenance yard is another. The location of
these is pretty constant and reasonably predictable and reasonably benign.
Dust devils on the other hand, while they may begin at convective
sources, are cyclonic whirlwinds that travel laterally across the ground,
sometimes for miles. DDs in our area are typically less than five meters in
diameter. One monster dust devil that went across a portable weather station
at the Naval Air Warfare Center spun the anemometer over 80 knots before it
ripped the mast apart. We watched that one travel about ten miles. On
another occasion, a monster went across a mobile home park and took the roof
off a home and dissassembled tool sheds like card houses.

[Marc Ramsey]

Casey Wilson wrote:
> Dust devils on the other hand, while they may begin at convective
> sources, are cyclonic whirlwinds that travel laterally across the ground,
> sometimes for miles. DDs in our area are typically less than five meters in
> diameter. One monster dust devil that went across a portable weather station
> at the Naval Air Warfare Center spun the anemometer over 80 knots before it
> ripped the mast apart. We watched that one travel about ten miles. On
> another occasion, a monster went across a mobile home park and took the roof
> off a home and dissassembled tool sheds like card houses.

Yeah, those are the kind of dust devils I look for 8^)

I remember one I saw in a valley north of Tonopah, when I was down low
looking for lift. It had one huge central column and six smaller ones
twisting around it. I could see huge pieces of sagebrush literally
getting blown out of its path. I pulled into it at roughly 1500 feet
AGL, and centered a 14 knot climb with dust and twigs flying all around
me. I was at 18000 feet in what seemed like a moment. Even at that
altitude, there was plenty of dust, and looking down into was like
looking into the maw of a huge snake that stretched all the way back
down to the ground.

You don't know what you're missing 8^)

Marc

[Dave Nadler YO]

I've seen this effect many times in Australia. Kept finding
lift over small irrigation ponds (altitude several hundred
feet). Decided to do an experiment whilst awaiting a buddy
who was interminable slow to get airborne. There was an
irrigation pond about 1km from the airfield. After losing
sufficient altitude doing aero, spoilered down to several
hundred feet over this pond, then climbed out and repeated
the procedure. Four or five times (like I said, he's slow).

The interesting features here (and in Arizona) are:
- very dry air, and
- shallow irrigation tank/pond subject to good heating (warm water)
Don't know that I understand the physics, but extremely
consistent.

Beaver Pond Lift is however a different phenom...

Best Wishes for 2004 to all, Dave "YO"


Peter Creswick > wrote in message >...
> Mike Rapoport wrote:
> >
> > The moisture doesn't really help lift until the air is fuly saturated and
> > starts condensing releasing heat. Also, the air above the water is cooled
> > by evaporation and is cooler than the surrounding air. I will never say
> > never and I don't dispute your or others experience, but the explanation
> > doesn't make sense to me.
> >
> > Mike
> > MU-2
> >
> > "Kirk Stant" > wrote in message
> > om...
> > > "K.P. Termaat" > wrote in message
> >...
> > > > My experience is that it works, especially on days with very low
> humidity,
> > > > but no boomers and only low.
> > > >
> > > >
> > > > "Mike Rapoport" > schreef in bericht
> > > > ink.net...
> > > > >
> > > > > You will find less lift over water of any kind, even if it is
> contained in
> > > > > vegetation. The best lift is always over the highest, dryest, darkest
> > > > > surface around. The water vapor idea is...well...it is hard to find a
> place
> > > > > to start...but it won't work
> > > > >
> > > > > Mike
> > > > > MU-2
> > > > >
> > >
> > > Have to disagree with you, Mike - out here in Arizona, in the desert
> > > areas that are not irrigated, we often find good lift directly over
> > > small cattle "tanks" - small shallow ponds that are scattered around.
> > > A lot of us have noticed this and compared notes, and it works; if too
> > > low to get to high, dark ground, I'll head for the nearest pond and it
> > > will usually turn up a nice thermal. We think it may be due to the
> > > fact that the ponds are in a natural low spot, and coupled with the
> > > little bit of moisture, could be the necessary trigger for a thermal.
> > >
> > > Now obviously, large irrigated farm fields or river basins are death
> > > to thermals - but a local lake (reservoir) seems to have little effect
> > > on thermal activity - could it be all the drunk boaters?
> > >
> > > What's the old saying about never saying never?
> > >
> > > Kirk
> > > LS6-b
>
> Have seen similar effects over the small dams on farms here too. My idea is that the air over the
> water cools by evaporating water out of the pond. In so doing it looses more heat and hence
> contracts more (gets denser) than it gains buoyancy by water vapour increase, ie, it gets both
> colder and denser overall than the surrounding surface air. As the dense pool of air becomes
> greater, it spreads out, ie, sort of collapses on itself, and pushes out over the edges of the pond
> / dam, particularly down slope over the dam wall, creating a miniature equivalent of a valley wind
> in the creek or down the slope, thus acting as a wedge trigger to lift the warm dry air off the
> ground.

[Chris Nicholas]

I can claim no specialist meteorological knowledge, but I believe the
following to be undeniable:

1. When a thermal forms, there is to some extent at least an inflow of
air, to prevent a vacuum forming under the rising airmass.

2. It is extremely unlikely that the inflow from every direction is
equal and symmetrical. If Coriolis effects are likely to be swamped by
local initial conditions for the inflow, the resultant rotation may be
in either direction.

3. It follows that there will be an element of rotation imparted to the
resulting thermal bubble/plume/call-it-what-you-will.

4. In some cases at least, the resultant rotation may be enough to be
noticeable.

5. During the inflow phase, any resultant rotation will speed up, by
conservation of angular momentum, like a dancer speeding up a spin by
pulling in the arms.

6. This rotation is certainly visible in dust devils, tornados, and
other smaller local eddies. Even in the UK's usually mild thermals, one
occasionally sees bits of straw, grass cuttings, or dead leaves picked
up and whirling round. In the days of stubble fires (farmers buring off
fields, after harvest - now banned) bits of burning straw could be seen
in the rotating thermals.

[I have used "rotation" in the layman's sense that the air and its
contents are going round. There is a technical use of the word, if I
recall my fluid dynamics correctly, that the water in a whirlpool has
"zero rotation" which may also apply in thermals, for the same reason -
it is a mechanism where things can go round quite fast without a
significant input of angular kinetic energy. It is a feature of a
rotating fluid mass where the middle is going round faster than the
outside, which happens in whirlpools and when you pull the plug out of a
circular basin full of water. I propose to leave it there. ]

Chris N.

[Mike Borgelt]

On Mon, 29 Dec 2003 10:24:40 GMT, "K.P. Termaat" > wrote:

>My experience is that it works, especially on days with very low humidity,
>but no boomers and only low.
>I'am talking about small shallow ponds in dry area's especially when the
>ponds are surrounded by sandy grounds with higher vegetation like trees. The
>buoyancy impuls from the evaporated water is apparently just good enough to
>start the thermal which then sucks air from its heated up vicinity.
>Has saved me many times when I was still flying my Pik20D or more recently
>my DG800S.
>
>Karel, NL
>V-2cxT
Water vapour has a molecular weight of a bit over 18 and dry air a bit
more than 28. Water vapour at the same pressure as the air around it
is considerably less dense than dry air. More water vapour= more
bouyancy.

Then again this may have more to do with low spots in the ground. I've
always found quarries (holes in the ground)to be excellent lift
sources when low.

Mike Borgelt

[Bert Willing]

I'll buy that one, JJ.

--
Bert Willing

ASW20 "TW"


"JJ Sinclair" > a écrit dans le message de
...
> >
> >I am not disputing the facts, I am disputing the explanation.
> >
>
> OK, Its time for my Minden pond theory. We have a small pond about 5 miles
east
> of the airport that consistently produces thermals. JJ's explanation;
Heated
> air that is slowely moving over the ground by the wind, suddenly comes to
the
> cooler pond edge. This cool edge of the pond acts as a trigger that forces
the
> heated air to break loose and start rising and that's why thermals can be
found
> at the edge of small ponds.
> JJ Sinclair

[Chris OCallaghan]

Eric,

The vortices at the edges of clouds I am referring to are in the
horizontal, though they typically have a significant vertical
component as well (otherwise, why bother with them). That's why it is
important to view them from directly below. This makes seeing the
horizontal component much easier.

Observed localized rotation at cloudbase (the only place we can see
it) is substantially lower than the rotation rate observed in dust
devils. This is expected. As we go up a dust devil, it expands with
altitude. Conservation of angular momentum alone will account for a
substantial reduction in rate of rotation.

As for the rate of rotation of the entire system, I have never
measured it. All I can say here and now is that it is slow but
observable. Let's, for the sake of argument, say that it moves three
times as fast as the minutehand on a clock, that is 18 degrees per
minute. (Remember, this is at the edges of the thermal. We would
expect increased rotation within a strong core.) If the cloud is 1/4m
in diameter, the speed of rotation is about 2 knots. That's a 4 knot
differential for a left versus right turn, with corresponding turn
radii for a given angle of bank.

Granted, the system is turbulent. And there are additional factors
that might contribute to large scale rotation such as wind shear,
inversion, perhaps even condensation.

For argument's sake, let's say that it does rotate, on both large and
localized scales. What advantage can we take? How can we detect it?
How might we change our approach, entry, and centering techniques to
maximize overall rate of climb? These are the questions worth
pondering.

[Kirk Stant]

Mike Borgelt > wrote in message >...

> Water vapour has a molecular weight of a bit over 18 and dry air a bit
> more than 28. Water vapour at the same pressure as the air around it
> is considerably less dense than dry air. More water vapour= more
> bouyancy.
>
> Then again this may have more to do with low spots in the ground. I've
> always found quarries (holes in the ground)to be excellent lift
> sources when low.

This discussion is fascinating. I've been flying gliders for some 27
years and have read a lot of books on the theory and practice (Moffat,
Reichmann, Piggott, etc) and never ran into any reference to this
thermal source (or trigger mechanism) - but here we have pilots from
three continents describing apparently the same, common, reliable
trigger mechanism - all apparently discovered empirically (thats how I
found it, that and following Andy around trying to keep up with him
:)) - Everyone always said head for the dry, high, dark ground, and
here are experienced pilots heading for a low pond!

Same thing with sandy areas - the books say to avoid them like the
plague, but the sandy washes here in Arizona are also consistent
thermal sources - and like the ponds/tanks, are low discontinuities in
the local terrain. In this case, I'm sure it's not the sand that is
causing the thermal, my uneducated guess is that the wash channels (or
collects) the incipient themal until it gets big and strong enough to
break loose.

Any Real Smart Guys out there care to give us a serious possible
explanation for these effects? - or maybe we need to keep this to
ourselves and let the youngsters figure it out for themselves! Got to
keep a few tricks in our bags, you know, something about age and
experience beating youth and skill...

Kirk

[Eric Greenwell]

Chris OCallaghan wrote:


> For argument's sake, let's say that it does rotate, on both large and
> localized scales. What advantage can we take? How can we detect it?

This will be a problem at the 18 deg/min you proposed. I circle at about
720 deg/min, so the chances of me noticing this while thermalling are
probably zero. For the small cloud you mentioned, the "observation time"
as I approached it would be something like 30 seconds or less, which
corresponds to a 9 degree rotation - hard to discern. Believe me, I have
intently watched thousands of cloud bottoms as I approached them, and I
have never noticed any large scale rotation. It might be there (like I
said, I'll try watching from the ground next time I see a cu), but it
isn't obvious.

> How might we change our approach, entry, and centering techniques to
> maximize overall rate of climb? These are the questions worth
> pondering.

Asking top competition pilots about this might be a way to start, though
I've never heard any of them mention rotation as a factor in their
decisions.
--
-----
change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

[db]

age and deception is even better



>ourselves and let the youngsters figure it out for themselves! Got to
>keep a few tricks in our bags, you know, something about age and
>experience beating youth and skill...
>
>Kirk

[Shawn Curry]

Chris OCallaghan wrote:

> Most people can't see a minute hand moving, but it moves nonetheless.
> It's a matter of patience. The problem with observing cloud rotation
> is that the cloud is constantly changing shape, therefore you cannot
> time lapse the same way you can when observing vertical development
> (or determining that a minute hand moves). As noted in my impolite
> post, you need to spend a half hour on your back. Then we can move on
> to discussing whether there's any real advantage to be had.

I've seen several time-lapse videos of cu. Never noticed (or had
pointed out) any rotation. I have seen mesocyclone footage. Very cool.
Do you know of any web sites with a clip showing this?

Cheers,
Shawn

[goneill]

When low I have always looked for the the point or line where the difference
in ground condition/temperature is likely to trigger uplift ,I have even had
the edge of a
small cloud shadow that I followed between 300-600ft agl for 10 miles
before finally enough warm air was encountered to form a decent enough
columm to climb consistantly
gary

"Kirk Stant" > wrote in message
om...
> Mike Borgelt > wrote in message
>...
>
> > Water vapour has a molecular weight of a bit over 18 and dry air a bit
> > more than 28. Water vapour at the same pressure as the air around it
> > is considerably less dense than dry air. More water vapour= more
> > bouyancy.
> >
> > Then again this may have more to do with low spots in the ground. I've
> > always found quarries (holes in the ground)to be excellent lift
> > sources when low.
>
> This discussion is fascinating. I've been flying gliders for some 27
> years and have read a lot of books on the theory and practice (Moffat,
> Reichmann, Piggott, etc) and never ran into any reference to this
> thermal source (or trigger mechanism) - but here we have pilots from
> three continents describing apparently the same, common, reliable
> trigger mechanism - all apparently discovered empirically (thats how I
> found it, that and following Andy around trying to keep up with him
> :)) - Everyone always said head for the dry, high, dark ground, and
> here are experienced pilots heading for a low pond!
>
> Same thing with sandy areas - the books say to avoid them like the
> plague, but the sandy washes here in Arizona are also consistent
> thermal sources - and like the ponds/tanks, are low discontinuities in
> the local terrain. In this case, I'm sure it's not the sand that is
> causing the thermal, my uneducated guess is that the wash channels (or
> collects) the incipient themal until it gets big and strong enough to
> break loose.
>
> Any Real Smart Guys out there care to give us a serious possible
> explanation for these effects? - or maybe we need to keep this to
> ourselves and let the youngsters figure it out for themselves! Got to
> keep a few tricks in our bags, you know, something about age and
> experience beating youth and skill...
>
> Kirk

[rolf hertenstein]

> Any Real Smart Guys out there care to give us a serious possible
> explanation for these effects? - or maybe we need to keep this to
> ourselves and let the youngsters figure it out for themselves! Got to
> keep a few tricks in our bags, you know, something about age and
> experience beating youth and skill...

Here's a hand-wavy guess. Whenever you have differences in the land
surface - like a pond next to solid ground - the heating is gonna be
different. The two differently heated parcels adjacent to each other
will tend to stir the air a bit. This can lead to a very local and
small-scale convergence which can act as the trigger. The bit of
water vapor from the air that was over the pond will help the average
buoyancy of the thermal - but I think the moisture effect is secondary
to the convergence.

I've no data to back this up; just a hunch. But whenever the
convergence has been noticable on a thermic day, that's where the best
thermals are.


Rolf (not a RSG, just guessing)

[Larry Dighera]

On Mon, 29 Dec 2003 02:02:43 GMT, "Casey Wilson" >
wrote in Message-Id: >:

>>>
>> >>Agreed. Circling direction is more often dictated by other gliders in
>> >>the thermal than meteorological phenomena and physics.
>> >>
>
> The protocol I was taught was that unless you are first into the
>thermal, you follow the left or right pattern of the gliders already there.
> I've never had a preference of right or left. I was taught to turn into
>whichever wingtip went up.

Turning into the rising wing is intuitive, and logical. I'm only able
to think of a couple of alternative techniques, but I would expect
neither of them to provide superior results.

> I was also taught that the most efficient technique, that is the
>highest rate of altitude gain, is in a 45-degree bank turn hopefully
>"coring" the thermal.

That is consistent with what has been written in the past in this
newsgroup concerning the optimum bank in a turn-back to the airport
maneuver.

> Up here in the Mojave Desert flying out of IYK, I've been in a couple of
>10 Knot thermals but 5 to 6 is the most common. I can't ever remember any
>kind of cyclonic rotation of any of them.
>
> That said, I did once, inadvertantly fly into a dust-devil. I NEVER want
>to do that again. If I had seen any dust indication that it was there I
>would definitely have avoided it in the first place.
>

My soaring experience was also in the Mojave Desert, Antelope Valley
area around El Mirage and toward the east and west of there. In the
summer, dust-devils were as plentiful as columns at the Forum. They
visibly marked areas above which the chance of encountering lift was
virtually assured.

I'd be interested in hearing more about your dust-devil encounter.

[Chris OCallaghan]

The few I've seen have always been at very low angles, focusing on
vertical development. I've not yet seen anything shot from directly
below.

Shawn Curry > wrote in message . net>...
> Chris OCallaghan wrote:
>
> > Most people can't see a minute hand moving, but it moves nonetheless.
> > It's a matter of patience. The problem with observing cloud rotation
> > is that the cloud is constantly changing shape, therefore you cannot
> > time lapse the same way you can when observing vertical development
> > (or determining that a minute hand moves). As noted in my impolite
> > post, you need to spend a half hour on your back. Then we can move on
> > to discussing whether there's any real advantage to be had.
>
> I've seen several time-lapse videos of cu. Never noticed (or had
> pointed out) any rotation. I have seen mesocyclone footage. Very cool.
> Do you know of any web sites with a clip showing this?
>
> Cheers,
> Shawn

[Chris OCallaghan]

Reichmann discusses this very briefly on page 5 of Cross Country
Soaring, under the heading 4) Instability Due to Moisture Differences.
Only one paragraph saying that it has been observed, but is not
technically measurable.

(Kirk Stant) wrote in message >...
> Mike Borgelt > wrote in message >...
>
> > Water vapour has a molecular weight of a bit over 18 and dry air a bit
> > more than 28. Water vapour at the same pressure as the air around it
> > is considerably less dense than dry air. More water vapour= more
> > bouyancy.
> >
> > Then again this may have more to do with low spots in the ground. I've
> > always found quarries (holes in the ground)to be excellent lift
> > sources when low.
>
> This discussion is fascinating. I've been flying gliders for some 27
> years and have read a lot of books on the theory and practice (Moffat,
> Reichmann, Piggott, etc) and never ran into any reference to this
> thermal source (or trigger mechanism) - but here we have pilots from
> three continents describing apparently the same, common, reliable
> trigger mechanism - all apparently discovered empirically (thats how I
> found it, that and following Andy around trying to keep up with him
> :)) - Everyone always said head for the dry, high, dark ground, and
> here are experienced pilots heading for a low pond!
>
> Same thing with sandy areas - the books say to avoid them like the
> plague, but the sandy washes here in Arizona are also consistent
> thermal sources - and like the ponds/tanks, are low discontinuities in
> the local terrain. In this case, I'm sure it's not the sand that is
> causing the thermal, my uneducated guess is that the wash channels (or
> collects) the incipient themal until it gets big and strong enough to
> break loose.
>
> Any Real Smart Guys out there care to give us a serious possible
> explanation for these effects? - or maybe we need to keep this to
> ourselves and let the youngsters figure it out for themselves! Got to
> keep a few tricks in our bags, you know, something about age and
> experience beating youth and skill...
>
> Kirk

[Adrian Jansen]

My somewhat uneducated guess is that any form of discontinuity is enough to
act as a trigger source for thermals. Doesnt really matter whether its
dark/light, high/low, dry/wet. Anything that breaks up the surface layer of
air warmed by the ground and starts any form of vertical motion will work.

--
Regards,

Adrian Jansen
J & K MicroSystems
Microcomputer solutions for industrial control

"Kirk Stant" > wrote in message
om...
> Mike Borgelt > wrote in message
>...
>
> > Water vapour has a molecular weight of a bit over 18 and dry air a bit
> > more than 28. Water vapour at the same pressure as the air around it
> > is considerably less dense than dry air. More water vapour= more
> > bouyancy.
> >
> > Then again this may have more to do with low spots in the ground. I've
> > always found quarries (holes in the ground)to be excellent lift
> > sources when low.
>
> This discussion is fascinating. I've been flying gliders for some 27
> years and have read a lot of books on the theory and practice (Moffat,
> Reichmann, Piggott, etc) and never ran into any reference to this
> thermal source (or trigger mechanism) - but here we have pilots from
> three continents describing apparently the same, common, reliable
> trigger mechanism - all apparently discovered empirically (thats how I
> found it, that and following Andy around trying to keep up with him
> :)) - Everyone always said head for the dry, high, dark ground, and
> here are experienced pilots heading for a low pond!
>
> Same thing with sandy areas - the books say to avoid them like the
> plague, but the sandy washes here in Arizona are also consistent
> thermal sources - and like the ponds/tanks, are low discontinuities in
> the local terrain. In this case, I'm sure it's not the sand that is
> causing the thermal, my uneducated guess is that the wash channels (or
> collects) the incipient themal until it gets big and strong enough to
> break loose.
>
> Any Real Smart Guys out there care to give us a serious possible
> explanation for these effects? - or maybe we need to keep this to
> ourselves and let the youngsters figure it out for themselves! Got to
> keep a few tricks in our bags, you know, something about age and
> experience beating youth and skill...
>
> Kirk

[K.P. Termaat]

Mike Borgelt wrote:
> Water vapour has a molecular weight of a bit over 18 and dry air a bit
> > more than 28. Water vapour at the same pressure as the air around it
> > is considerably less dense than dry air. More water vapour= more
> > bouyancy.

Just a simple approach with rough figures to support Mike's statement and
hopefully to trigger the "smart guys".
At atmospheric pressure (say 1013 hPa) and at 20 C° the density of dry air
is about 1.22 kg/m3. Pure water vapor at atmospheric pressure has a density
of 18/28 x 1.22 = 0.785 kg/m3, or 785 g/m3.
Air is saturated with water vapor when it contains 25 g/m3 at 20 C°.
Assume a relative humidity of say 30% on a dry day. Then one cubic meter of
air contains 0.3 x 25 = 7.5 g of water vapor and the air has then a density
of 1.2159 kg/m3. Assume further that over a shallow pond the humidity of the
air increases to 60% due to a serious evaporation from the pond. Then the
air directly over the pond will contain 0.6 x 25 = 15.0 g/m3 corresponding
to an air density of 1.2118 kg/m3.
So one cubic meter of air having 60% humidity is 1.2159 - 1.2118= 0.0041 kg
lighter then air with a humidity of 30%. This 4.1 g/m3 does not look much,
but compare this figure with the decrease in density when air is heated up.
The temperature coëfficiënt of air is 0.0044 kg/m3 per °C at 20 °C, meaning
that when air is heated up by one degree its density decreases with 4.4
g/m3.
So one may conclude that changing the relative humidity of air from 30% to
60% has the same effect on buoyancy as raising the temperature of air by 1
°C.
So it may be worthwhile indeed to search for a thermal over a shallow pond
in a dry area when low as I stated earlier.

Karel, NL


"Kirk Stant" > schreef in bericht
om...
>
> This discussion is fascinating. I've been flying gliders for some 27
> years and have read a lot of books on the theory and practice (Moffat,
> Reichmann, Piggott, etc) and never ran into any reference to this
> thermal source (or trigger mechanism) - but here we have pilots from
> three continents describing apparently the same, common, reliable
> trigger mechanism - all apparently discovered empirically (thats how I
> found it, that and following Andy around trying to keep up with him
> :)) - Everyone always said head for the dry, high, dark ground, and
> here are experienced pilots heading for a low pond!
>
> Same thing with sandy areas - the books say to avoid them like the
> plague, but the sandy washes here in Arizona are also consistent
> thermal sources - and like the ponds/tanks, are low discontinuities in
> the local terrain. In this case, I'm sure it's not the sand that is
> causing the thermal, my uneducated guess is that the wash channels (or
> collects) the incipient themal until it gets big and strong enough to
> break loose.
>
> Any Real Smart Guys out there care to give us a serious possible
> explanation for these effects? - or maybe we need to keep this to
> ourselves and let the youngsters figure it out for themselves! Got to
> keep a few tricks in our bags, you know, something about age and
> experience beating youth and skill...
>
> Kirk

[Bob Salvo]

Warm breeze picks up moisture at upwing edge of pond. Warm moist air being
lighter than dry warm air, begins to rise, initiating thermal.

Happy New Year!
Bob
Bob

[Mike Borgelt]

On Tue, 30 Dec 2003 23:35:20 GMT, "K.P. Termaat" > wrote:

>Mike Borgelt wrote:
>> Water vapour has a molecular weight of a bit over 18 and dry air a bit
>> > more than 28. Water vapour at the same pressure as the air around it
>> > is considerably less dense than dry air. More water vapour= more
>> > bouyancy.
>
>Just a simple approach with rough figures to support Mike's statement and
>hopefully to trigger the "smart guys".
>At atmospheric pressure (say 1013 hPa) and at 20 C° the density of dry air
>is about 1.22 kg/m3. Pure water vapor at atmospheric pressure has a density
>of 18/28 x 1.22 = 0.785 kg/m3, or 785 g/m3.
>Air is saturated with water vapor when it contains 25 g/m3 at 20 C°.
>Assume a relative humidity of say 30% on a dry day. Then one cubic meter of
>air contains 0.3 x 25 = 7.5 g of water vapor and the air has then a density
>of 1.2159 kg/m3. Assume further that over a shallow pond the humidity of the
>air increases to 60% due to a serious evaporation from the pond. Then the
>air directly over the pond will contain 0.6 x 25 = 15.0 g/m3 corresponding
>to an air density of 1.2118 kg/m3.
>So one cubic meter of air having 60% humidity is 1.2159 - 1.2118= 0.0041 kg
>lighter then air with a humidity of 30%. This 4.1 g/m3 does not look much,
>but compare this figure with the decrease in density when air is heated up.
>The temperature coëfficiënt of air is 0.0044 kg/m3 per °C at 20 °C, meaning
>that when air is heated up by one degree its density decreases with 4.4
>g/m3.
>So one may conclude that changing the relative humidity of air from 30% to
>60% has the same effect on buoyancy as raising the temperature of air by 1
>°C.
>So it may be worthwhile indeed to search for a thermal over a shallow pond
>in a dry area when low as I stated earlier.
>
>Karel, NL
Thanks for that Karel. It is always nice to put some numbers on the
arm waving.

Mike Borgelt

[K.P. Termaat]

"Bob Salvo" > schreef in bericht
...
> Warm breeze picks up moisture at upwing edge of pond. Warm moist air
being
> lighter than dry warm air, begins to rise, initiating thermal.
>
> Happy New Year!
> Bob

Yes, I agree Bob, Karel, NL

Mike Borgelt wrote:
> Water vapour has a molecular weight of a bit over 18 and dry air a bit
> > more than 28. Water vapour at the same pressure as the air around it
> > is considerably less dense than dry air. More water vapour= more
> > bouyancy.

Just a simple approach with rough figures to support Mike's statement and
hopefully to trigger the "smart guys".
At atmospheric pressure (say 1013 hPa) and at 20 C° the density of dry air
is about 1.22 kg/m3. Pure water vapor at atmospheric pressure has a density
of 18/28 x 1.22 = 0.785 kg/m3, or 785 g/m3.
Air is saturated with water vapor when it contains 25 g/m3 at 20 C°.
Assume a relative humidity of say 30% on a dry day. Then one cubic meter of
air contains 0.3 x 25 = 7.5 g of water vapor and the air has then a density
of 1.2159 kg/m3. Assume further that over a shallow pond the humidity of the
air increases to 60% due to a serious evaporation from the pond. Then the
air directly over the pond will contain 0.6 x 25 = 15.0 g/m3 corresponding
to an air density of 1.2118 kg/m3.
So one cubic meter of air having 60% humidity is 1.2159 - 1.2118= 0.0041 kg
lighter then air with a humidity of 30%. This 4.1 g/m3 does not look much,
but compare this figure with the decrease in density when air is heated up.
The temperature coëfficiënt of air is 0.0044 kg/m3 per °C at 20 °C, meaning
that when air is heated up by one degree its density decreases with 4.4
g/m3.
So one may conclude that changing the relative humidity of air from 30% to
60% has the same effect on buoyancy as raising the temperature of air by 1
°C.
So it may be worthwhile indeed to search for a thermal over a shallow pond
in a dry area when low as I stated earlier.

Karel, NL

[Andy Durbin]

"K.P. Termaat" > wrote
>
> Just a simple approach with rough figures to support Mike's statement and
> hopefully to trigger the "smart guys".
> At atmospheric pressure (say 1013 hPa) and at 20 C° the density of dry air
> is about 1.22 kg/m3. Pure water vapor at atmospheric pressure has a density
> of 18/28 x 1.22 = 0.785 kg/m3, or 785 g/m3.
> Air is saturated with water vapor when it contains 25 g/m3 at 20 C°.
> Assume a relative humidity of say 30% on a dry day. Then one cubic meter of
> air contains 0.3 x 25 = 7.5 g of water vapor and the air has then a density
> of 1.2159 kg/m3. Assume further that over a shallow pond the humidity of the
> air increases to 60% due to a serious evaporation from the pond. Then the
> air directly over the pond will contain 0.6 x 25 = 15.0 g/m3 corresponding
> to an air density of 1.2118 kg/m3.
> So one cubic meter of air having 60% humidity is 1.2159 - 1.2118= 0.0041 kg
> lighter then air with a humidity of 30%. This 4.1 g/m3 does not look much,
> but compare this figure with the decrease in density when air is heated up.
> The temperature coëfficiënt of air is 0.0044 kg/m3 per °C at 20 °C, meaning
> that when air is heated up by one degree its density decreases with 4.4
> g/m3.
> So one may conclude that changing the relative humidity of air from 30% to
> 60% has the same effect on buoyancy as raising the temperature of air by 1
> °C.
> So it may be worthwhile indeed to search for a thermal over a shallow pond
> in a dry area when low as I stated earlier.
>
> Karel, NL


I don't know how it influences the analysis but, for Arizona, ambient
temp of 40 plus deg C and ambient humidity of about 15 percent are
more typical than the figures you used. Actual surface temperatures
probably run close to 60 C on a hot day.

I agree with others that the humidity concontinuity is probably the
trigger mechanism. Once the thermal has started it pulls in all the
surrounding super heated dry air.


Andy (GY)

[K.P. Termaat]

"Andy Durbin" > schreef in bericht
om...
> "K.P. Termaat" > wrote
> >
> > Just a simple approach with rough figures to support Mike's statement
and
> > hopefully to trigger the "smart guys".
> > At atmospheric pressure (say 1013 hPa) and at 20 C° the density of dry
air
> > is about 1.22 kg/m3. Pure water vapor at atmospheric pressure has a
density
> > of 18/28 x 1.22 = 0.785 kg/m3, or 785 g/m3.
> > Air is saturated with water vapor when it contains 25 g/m3 at 20 C°.
> > Assume a relative humidity of say 30% on a dry day. Then one cubic meter
of
> > air contains 0.3 x 25 = 7.5 g of water vapor and the air has then a
density
> > of 1.2159 kg/m3. Assume further that over a shallow pond the humidity of
the
> > air increases to 60% due to a serious evaporation from the pond. Then
the
> > air directly over the pond will contain 0.6 x 25 = 15.0 g/m3
corresponding
> > to an air density of 1.2118 kg/m3.
> > So one cubic meter of air having 60% humidity is 1.2159 - 1.2118= 0.0041
kg
> > lighter then air with a humidity of 30%. This 4.1 g/m3 does not look
much,
> > but compare this figure with the decrease in density when air is heated
up.
> > The temperature coëfficiënt of air is 0.0044 kg/m3 per °C at 20 °C,
meaning
> > that when air is heated up by one degree its density decreases with 4.4
> > g/m3.
> > So one may conclude that changing the relative humidity of air from 30%
to
> > 60% has the same effect on buoyancy as raising the temperature of air
by 1
> > °C.
> > So it may be worthwhile indeed to search for a thermal over a shallow
pond
> > in a dry area when low as I stated earlier.
> >
> > Karel, NL
>
>
> I don't know how it influences the analysis but, for Arizona, ambient
> temp of 40 plus deg C and ambient humidity of about 15 percent are
> more typical than the figures you used. Actual surface temperatures
> probably run close to 60 C on a hot day.
>
> I agree with others that the humidity concontinuity is probably the
> trigger mechanism. Once the thermal has started it pulls in all the
> surrounding super heated dry air.
>
>
> Andy (GY)

The additional buoyance of the air over the pond is caused by the change in
humidity of this air. In my example I used a change of 30% in humidity
causing an equal effect as heating up the air by an additional one degree C.
With very low humidity to start with, e.g. the 15% you mention, it may be
possible that the change in humidity is more then 30% causing a somewhat
larger decrease in air density. The higher ambient temperatures have also a
positive effect on this, so without going through the calculations once
more it looks like the water vapor effect is stronger for your Arizona case.

Karel, NL

[Ken Kochanski]

This article from Weatherwise looks at the mechanisms that cause spin
in storms, dust devils, etc. The thermals we fly in typically form in
the high following a frontal passage ... the flow in a high is
clockwise ... could it cause most thermals to have a clockwise
rotations ?

http://www.weatherwise.org/qr/qry.02coriolistorn.html


(Chris OCallaghan) wrote in message >...
> The few I've seen have always been at very low angles, focusing on
> vertical development. I've not yet seen anything shot from directly
> below.
>
> Shawn Curry > wrote in message . net>...
> > Chris OCallaghan wrote:
> >
> > > Most people can't see a minute hand moving, but it moves nonetheless.
> > > It's a matter of patience. The problem with observing cloud rotation
> > > is that the cloud is constantly changing shape, therefore you cannot
> > > time lapse the same way you can when observing vertical development
> > > (or determining that a minute hand moves). As noted in my impolite
> > > post, you need to spend a half hour on your back. Then we can move on
> > > to discussing whether there's any real advantage to be had.
> >
> > I've seen several time-lapse videos of cu. Never noticed (or had
> > pointed out) any rotation. I have seen mesocyclone footage. Very cool.
> > Do you know of any web sites with a clip showing this?
> >
> > Cheers,
> > Shawn

[Bill Daniels]

"Ken Kochanski" > wrote in message
om...
> This article from Weatherwise looks at the mechanisms that cause spin
> in storms, dust devils, etc. The thermals we fly in typically form in
> the high following a frontal passage ... the flow in a high is
> clockwise ... could it cause most thermals to have a clockwise
> rotations ?
>
> http://www.weatherwise.org/qr/qry.02coriolistorn.html
>
>
Alas, there have been studies that have found an almost even population of
left and right hand rotation with, perhaps, a small edge to the left hand
rotation in the northern hemisphere. Coriolis effects are more likely to be
seen on large scales - much larger than dust devils.

On one occasion I observed a very large dust devil over a dry lake in
California. The central thermal was rotating counter-clockwise but ringed
by a dozen or more dust devils rotating clockwise in the shear layer at the
edge of the large one - somewhat like planet gears around a sun gear. The
smaller dust devils were more obvious than the large central one so a casual
ground observer would think that the majority of dust devils that day were
clockwise. You had to be airborne to see the larger pattern. It pays to be
careful with observations.

When you can determine the direction of rotation from airborne trash or
dust, it pays to turn against it.

Bill Daniels

[Ken Kochanski]

Bill,

The Weatherwise article also supports the view that thermals (if you
consider them weaker cousins of dust devils) have equal probability of
left or right spin.

"To summarize, the Coriolis force has little bearing on the sense of
rotation in dust devils--about half of them spin one way and half the
other. By contrast, the large-scale rotation in the vicinity of
tornadoes and the storms that spawn them is usually cyclonic,
influenced by the Coriolis force."

But, how much advantage will you get choosing the correct thermalling
direction ? Let's assume the thermal is 500' in diameter with a
uniform lift of 5 knots and the rotation speed at the 250' radius is
10 MPH.

KK


"Bill Daniels" > wrote in message . net>...
> "Ken Kochanski" > wrote in message
> om...
> > This article from Weatherwise looks at the mechanisms that cause spin
> > in storms, dust devils, etc. The thermals we fly in typically form in
> > the high following a frontal passage ... the flow in a high is
> > clockwise ... could it cause most thermals to have a clockwise
> > rotations ?
> >
> > http://www.weatherwise.org/qr/qry.02coriolistorn.html
> >
> >
> Alas, there have been studies that have found an almost even population of
> left and right hand rotation with, perhaps, a small edge to the left hand
> rotation in the northern hemisphere. Coriolis effects are more likely to be
> seen on large scales - much larger than dust devils.
>
> On one occasion I observed a very large dust devil over a dry lake in
> California. The central thermal was rotating counter-clockwise but ringed
> by a dozen or more dust devils rotating clockwise in the shear layer at the
> edge of the large one - somewhat like planet gears around a sun gear. The
> smaller dust devils were more obvious than the large central one so a casual
> ground observer would think that the majority of dust devils that day were
> clockwise. You had to be airborne to see the larger pattern. It pays to be
> careful with observations.
>
> When you can determine the direction of rotation from airborne trash or
> dust, it pays to turn against it.
>
> Bill Daniels

[Bill Daniels]

KK

If there is a uniform lift distribution, and the thermal large enough, there
would be little advantage to one direction of turn over another. However,
if there is a small, strong core then turning against the rotation would
reduce the turn radius and make it easier to stay in the core. At least
that's how it seems to work for me.

Bill Daniels

"Ken Kochanski" > wrote in message
om...
> Bill,
>
> The Weatherwise article also supports the view that thermals (if you
> consider them weaker cousins of dust devils) have equal probability of
> left or right spin.
>
> "To summarize, the Coriolis force has little bearing on the sense of
> rotation in dust devils--about half of them spin one way and half the
> other. By contrast, the large-scale rotation in the vicinity of
> tornadoes and the storms that spawn them is usually cyclonic,
> influenced by the Coriolis force."
>
> But, how much advantage will you get choosing the correct thermalling
> direction ? Let's assume the thermal is 500' in diameter with a
> uniform lift of 5 knots and the rotation speed at the 250' radius is
> 10 MPH.
>
> KK
>
>
> "Bill Daniels" > wrote in message
. net>...
> > "Ken Kochanski" > wrote in message
> > om...
> > > This article from Weatherwise looks at the mechanisms that cause spin
> > > in storms, dust devils, etc. The thermals we fly in typically form in
> > > the high following a frontal passage ... the flow in a high is
> > > clockwise ... could it cause most thermals to have a clockwise
> > > rotations ?
> > >
> > > http://www.weatherwise.org/qr/qry.02coriolistorn.html
> > >
> > >
> > Alas, there have been studies that have found an almost even population
of
> > left and right hand rotation with, perhaps, a small edge to the left
hand
> > rotation in the northern hemisphere. Coriolis effects are more likely
to be
> > seen on large scales - much larger than dust devils.
> >
> > On one occasion I observed a very large dust devil over a dry lake in
> > California. The central thermal was rotating counter-clockwise but
ringed
> > by a dozen or more dust devils rotating clockwise in the shear layer at
the
> > edge of the large one - somewhat like planet gears around a sun gear.
The
> > smaller dust devils were more obvious than the large central one so a
casual
> > ground observer would think that the majority of dust devils that day
were
> > clockwise. You had to be airborne to see the larger pattern. It pays
to be
> > careful with observations.
> >
> > When you can determine the direction of rotation from airborne trash or
> > dust, it pays to turn against it.
> >
> > Bill Daniels

[Anonymous Anonymous]

Many years ago when I was being taught to thermal,
ALL of my instructors here in the UK taught me to sense
which wing pitched up and to turn in that direction
in order to find and centre the thermal (subject to
no other glider already being established in the thermal
of course).

I would imagine that any benefit achieved by turning
against any airmass rotation in the thermal would be
greatly outweighed by turning into the thermal towards
the core in the initial turn and then sticking with
that direction (subject to the first turn being in
the correct direction).

My instructors also used to watch for any bias students
might show in their turns in order to ensure that they
are comfortably able to turn in either direction (and
therefore centre thermals as fast as possible).

I would imagine that any pilot who has a 'preferred'
turning direction (either becasue they like turning
in that direction or because they believe they will
benefit from some rotational effect) is going to be
slower to cente a thermal compared to a pilot of similar
ability who has no such preference.

Happy New Year





At 15:24 01 January 2004, Bill Daniels wrote:
>KK
>
>If there is a uniform lift distribution, and the thermal
>large enough, there
>would be little advantage to one direction of turn
>over another. However,
>if there is a small, strong core then turning against
>the rotation would
>reduce the turn radius and make it easier to stay in
>the core. At least
>that's how it seems to work for me.
>
>Bill Daniels
>
>'Ken Kochanski' wrote in message
om...
>> Bill,
>>
>> The Weatherwise article also supports the view that
>>thermals (if you
>> consider them weaker cousins of dust devils) have
>>equal probability of
>> left or right spin.
>>
>> 'To summarize, the Coriolis force has little bearing
>>on the sense of
>> rotation in dust devils--about half of them spin one
>>way and half the
>> other. By contrast, the large-scale rotation in the
>>vicinity of
>> tornadoes and the storms that spawn them is usually
>>cyclonic,
>> influenced by the Coriolis force.'
>>
>> But, how much advantage will you get choosing the
>>correct thermalling
>> direction ? Let's assume the thermal is 500' in diameter
>>with a
>> uniform lift of 5 knots and the rotation speed at
>>the 250' radius is
>> 10 MPH.
>>
>> KK
>>
>>
>> 'Bill Daniels' wrote in message
>news:...
>> > 'Ken Kochanski' wrote in message
>> > om...
>> > > This article from Weatherwise looks at the mechanisms
>>>>that cause spin
>> > > in storms, dust devils, etc. The thermals we fly
>>>>in typically form in
>> > > the high following a frontal passage ... the flow
>>>>in a high is
>> > > clockwise ... could it cause most thermals to have
>>>>a clockwise
>> > > rotations ?
>> > >
>> > > http://www.weatherwise.org/qr/qry.02coriolistorn.html
>> > >
>> > >
>> > Alas, there have been studies that have found an
>>>almost even population
>of
>> > left and right hand rotation with, perhaps, a small
>>>edge to the left
>hand
>> > rotation in the northern hemisphere. Coriolis effects
>>>are more likely
>to be
>> > seen on large scales - much larger than dust devils.
>> >
>> > On one occasion I observed a very large dust devil
>>>over a dry lake in
>> > California. The central thermal was rotating counter-clockwise
>>>>but
>ringed
>> > by a dozen or more dust devils rotating clockwise
>>>in the shear layer at
>the
>> > edge of the large one - somewhat like planet gears
>>>around a sun gear.
>The
>> > smaller dust devils were more obvious than the large
>>>central one so a
>casual
>> > ground observer would think that the majority of
>>>dust devils that day
>were
>> > clockwise. You had to be airborne to see the larger
>>>pattern. It pays
>to be
>> > careful with observations.
>> >
>> > When you can determine the direction of rotation
>>>from airborne trash or
>> > dust, it pays to turn against it.
>> >
>> > Bill Daniels
>
>

[Eric Greenwell]

Anonymous Anonymous wrote:

> I would imagine that any pilot who has a 'preferred'
> turning direction (either becasue they like turning
> in that direction or because they believe they will
> benefit from some rotational effect) is going to be
> slower to cente a thermal compared to a pilot of similar
> ability who has no such preference.

Perhaps, but I go with my preference only when there is no wing lift or
other indication (about 50% of the time), the most common situation I
encounter; consequently, about 75% of my circling is to the right,
unless I make an effort to do otherwise, such as before a contest.
--
-----
change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

[F.L. Whiteley]

"Anonymous Anonymous" > wrote in
message ...
> Many years ago when I was being taught to thermal,
> ALL of my instructors here in the UK taught me to sense
> which wing pitched up and to turn in that direction
> in order to find and centre the thermal (subject to
> no other glider already being established in the thermal
> of course).
>
> I would imagine that any benefit achieved by turning
> against any airmass rotation in the thermal would be
> greatly outweighed by turning into the thermal towards
> the core in the initial turn and then sticking with
> that direction (subject to the first turn being in
> the correct direction).
>
> My instructors also used to watch for any bias students
> might show in their turns in order to ensure that they
> are comfortably able to turn in either direction (and
> therefore centre thermals as fast as possible).
>
> I would imagine that any pilot who has a 'preferred'
> turning direction (either becasue they like turning
> in that direction or because they believe they will
> benefit from some rotational effect) is going to be
> slower to cente a thermal compared to a pilot of similar
> ability who has no such preference.
>
Although I was taught the same in the UK, there are other techniques. One
method suggests that you will center more quickly by turning away from the
lifted wing and find the core 270degs later and that by
turning toward the lifted wing you will fish around for the core longer.

Frank Whiteley

[Mike Lindsay]

In article >, K.P. Termaat
> writes
>
>"Bob Salvo" > schreef in bericht
...
>> Warm breeze picks up moisture at upwing edge of pond. Warm moist air
>being
>> lighter than dry warm air, begins to rise, initiating thermal.
>>
>> Happy New Year!
>> Bob
>
>Yes, I agree Bob, Karel, NL
>
>Mike Borgelt wrote:
>> Water vapour has a molecular weight of a bit over 18 and dry air a bit
>> > more than 28. Water vapour at the same pressure as the air around it
>> > is considerably less dense than dry air. More water vapour= more
>> > bouyancy.
>
>Just a simple approach with rough figures to support Mike's statement and
>hopefully to trigger the "smart guys".
>At atmospheric pressure (say 1013 hPa) and at 20 C the density of dry air
>is about 1.22 kg/m3. Pure water vapor at atmospheric pressure has a density
>of 18/28 x 1.22 = 0.785 kg/m3, or 785 g/m3.
>Air is saturated with water vapor when it contains 25 g/m3 at 20 C°.
>Assume a relative humidity of say 30% on a dry day. Then one cubic meter of
>air contains 0.3 x 25 = 7.5 g of water vapor and the air has then a density
>of 1.2159 kg/m3. Assume further that over a shallow pond the humidity of the
>air increases to 60% due to a serious evaporation from the pond. Then the
>air directly over the pond will contain 0.6 x 25 = 15.0 g/m3 corresponding
>to an air density of 1.2118 kg/m3.
>So one cubic meter of air having 60% humidity is 1.2159 - 1.2118= 0.0041 kg
>lighter then air with a humidity of 30%. This 4.1 g/m3 does not look much,
>but compare this figure with the decrease in density when air is heated up.
>The temperature coëfficiënt of air is 0.0044 kg/m3 per °C at 20 °C, meaning
>that when air is heated up by one degree its density decreases with 4.4
>g/m3.
>So one may conclude that changing the relative humidity of air from 30% to
>60% has the same effect on buoyancy as raising the temperature of air by 1
>°C.
>So it may be worthwhile indeed to search for a thermal over a shallow pond
>in a dry area when low as I stated earlier.
>
>Karel, NL
>
But wouldn't the latent heat of evaporation cool the air more that the
1deg C?. In which case a pond wouldn't work. But WTHDIK?

About 15 miles east of our site there is a low-lying marshland area
about 40 miles across which is all cut up with rivers and drainage
canals. I remember reading in an soaring text of the 1970s (I think it
was New Soaring Pilot) that it was a good idea to avoid this area
because all that water would stop convection.

So I asked on of the most experienced club members about it; he said
he'd not had any difficulty finding thermals there. He should know, he
had several UK records.

>
>
>
>

--
Mike Lindsay

[Bill Daniels]

"Mike Lindsay" > wrote in message
...
> In article >, K.P. Termaat
> > writes
> >
> >"Bob Salvo" > schreef in bericht
> ...
> >> Warm breeze picks up moisture at upwing edge of pond. Warm moist air
> >being
> >> lighter than dry warm air, begins to rise, initiating thermal.
> >>
> >> Happy New Year!
> >> Bob
> >
> >Yes, I agree Bob, Karel, NL
> >
> >Mike Borgelt wrote:
> >> Water vapour has a molecular weight of a bit over 18 and dry air a bit
> >> > more than 28. Water vapour at the same pressure as the air around it
> >> > is considerably less dense than dry air. More water vapour= more
> >> > bouyancy.
> >
> >Just a simple approach with rough figures to support Mike's statement and
> >hopefully to trigger the "smart guys".
> >At atmospheric pressure (say 1013 hPa) and at 20 C the density of dry air
> >is about 1.22 kg/m3. Pure water vapor at atmospheric pressure has a
density
> >of 18/28 x 1.22 = 0.785 kg/m3, or 785 g/m3.
> >Air is saturated with water vapor when it contains 25 g/m3 at 20 C°.
> >Assume a relative humidity of say 30% on a dry day. Then one cubic meter
of
> >air contains 0.3 x 25 = 7.5 g of water vapor and the air has then a
density
> >of 1.2159 kg/m3. Assume further that over a shallow pond the humidity of
the
> >air increases to 60% due to a serious evaporation from the pond. Then the
> >air directly over the pond will contain 0.6 x 25 = 15.0 g/m3
corresponding
> >to an air density of 1.2118 kg/m3.
> >So one cubic meter of air having 60% humidity is 1.2159 - 1.2118= 0.0041
kg
> >lighter then air with a humidity of 30%. This 4.1 g/m3 does not look
much,
> >but compare this figure with the decrease in density when air is heated
up.
> >The temperature coëfficiënt of air is 0.0044 kg/m3 per °C at 20 °C,
meaning
> >that when air is heated up by one degree its density decreases with 4.4
> >g/m3.
> >So one may conclude that changing the relative humidity of air from 30%
to
> >60% has the same effect on buoyancy as raising the temperature of air by
1
> >°C.
> >So it may be worthwhile indeed to search for a thermal over a shallow
pond
> >in a dry area when low as I stated earlier.
> >
> >Karel, NL
> >
> But wouldn't the latent heat of evaporation cool the air more that the
> 1deg C?. In which case a pond wouldn't work. But WTHDIK?
>
> About 15 miles east of our site there is a low-lying marshland area
> about 40 miles across which is all cut up with rivers and drainage
> canals. I remember reading in an soaring text of the 1970s (I think it
> was New Soaring Pilot) that it was a good idea to avoid this area
> because all that water would stop convection.
>
> So I asked on of the most experienced club members about it; he said
> he'd not had any difficulty finding thermals there. He should know, he
> had several UK records.

> --
> Mike Lindsay

This supports something I saw back in the 1960's from instrumented airplane
traverses made at mid levels in strongly convective conditions . The data
clearly showed the updrafts corresponding to thermals but did not show any
temperature rise in the thermals. Instead, they showed an increase in
absolute humidity corresponding to an increase of about 30% in relative
humidity over the surrounding air.

It's easy to see that the relative humidity in a thermal steadily increases
with height above the ground until it reaches 100% at cloud base. The
source of most of this moisture has to be the earth's surface below so the
thermal is a transport mechanism that lifts water vapor up to cloud base.

This has always led me to think that people looking to invent remote thermal
sensors should not be looking for water vapor and not warm air. Water vapor
has interesting infrared absorption spectra that might allow IR Imaging of
thermals (it sure works well in weather satellite images). Wingtip mounted
wet bulb sensors would directly read the temperature + humidity which should
correspond nicely to the spanwise buoyancy gradient and should be a reliable
indicator of the best direction of turn when entering a thermal.

Bill Daniels

[K.P. Termaat]

Hi Mike,

The latent heat of evaporation comes to the account of the water in the
pool. So no problem here.
Looking for thermals in a marshland with quite some water in it is not such
a good idea on blue days. The layer of air close to the ground just does not
heat up enough to become unstable looks like. However with some "hot spots"
and unstable meteo conditions there need not be a problem in forming
thermals over marshland, but usually these areas should better be avoided is
also my experience.

Karel, NL

"Mike Lindsay" > schreef in bericht
...
> In article >, K.P. Termaat
> > writes
> >
> >"Bob Salvo" > schreef in bericht
> ...
> >> Warm breeze picks up moisture at upwing edge of pond. Warm moist air
> >being
> >> lighter than dry warm air, begins to rise, initiating thermal.
> >>
> >> Happy New Year!
> >> Bob
> >
> >Yes, I agree Bob, Karel, NL
> >
> >Mike Borgelt wrote:
> >> Water vapour has a molecular weight of a bit over 18 and dry air a bit
> >> > more than 28. Water vapour at the same pressure as the air around it
> >> > is considerably less dense than dry air. More water vapour= more
> >> > bouyancy.
> >
> >Just a simple approach with rough figures to support Mike's statement and
> >hopefully to trigger the "smart guys".
> >At atmospheric pressure (say 1013 hPa) and at 20 C the density of dry air
> >is about 1.22 kg/m3. Pure water vapor at atmospheric pressure has a
density
> >of 18/28 x 1.22 = 0.785 kg/m3, or 785 g/m3.
> >Air is saturated with water vapor when it contains 25 g/m3 at 20 C°.
> >Assume a relative humidity of say 30% on a dry day. Then one cubic meter
of
> >air contains 0.3 x 25 = 7.5 g of water vapor and the air has then a
density
> >of 1.2159 kg/m3. Assume further that over a shallow pond the humidity of
the
> >air increases to 60% due to a serious evaporation from the pond. Then the
> >air directly over the pond will contain 0.6 x 25 = 15.0 g/m3
corresponding
> >to an air density of 1.2118 kg/m3.
> >So one cubic meter of air having 60% humidity is 1.2159 - 1.2118= 0.0041
kg
> >lighter then air with a humidity of 30%. This 4.1 g/m3 does not look
much,
> >but compare this figure with the decrease in density when air is heated
up.
> >The temperature coëfficiënt of air is 0.0044 kg/m3 per °C at 20 °C,
meaning
> >that when air is heated up by one degree its density decreases with 4.4
> >g/m3.
> >So one may conclude that changing the relative humidity of air from 30%
to
> >60% has the same effect on buoyancy as raising the temperature of air by
1
> >°C.
> >So it may be worthwhile indeed to search for a thermal over a shallow
pond
> >in a dry area when low as I stated earlier.
> >
> >Karel, NL
> >
> But wouldn't the latent heat of evaporation cool the air more that the
> 1deg C?. In which case a pond wouldn't work. But WTHDIK?
>
> About 15 miles east of our site there is a low-lying marshland area
> about 40 miles across which is all cut up with rivers and drainage
> canals. I remember reading in an soaring text of the 1970s (I think it
> was New Soaring Pilot) that it was a good idea to avoid this area
> because all that water would stop convection.
>
> So I asked on of the most experienced club members about it; he said
> he'd not had any difficulty finding thermals there. He should know, he
> had several UK records.
>
> >
> >
> >
> >
>
> --
> Mike Lindsay

[K.P. Termaat]

Hi Bill,

The observations you mention are very interesting and your remark that it is
better to look for moisture rather then for temperature in detecting
thermals is certainly true I think.

Using a forward looking IR sensor and a proper means of display in a glider
may be an idea to work on. I am not sure wings are long enough to detect a
usable spanwise buoyancy.
I remember reading about detecting spanwise temperature gradiënts by the
Germans. As a matter of fact I used very sensitive thermocouples on my
Pik-20D several years ago to detect a direction to turn into when hitting
unstable air. However this was quite unsuccesfull and supports your idea
that it is better to look for humidity rather then temperature to locate
upgoing drafts.

Karel, NL


"Bill Daniels" > schreef in bericht
link.net...
>
> "Mike Lindsay" > wrote in message
> ...
> > In article >, K.P. Termaat
> > > writes
> > >
> > >"Bob Salvo" > schreef in bericht
> > ...
> > >> Warm breeze picks up moisture at upwing edge of pond. Warm moist air
> > >being
> > >> lighter than dry warm air, begins to rise, initiating thermal.
> > >>
> > >> Happy New Year!
> > >> Bob
> > >
> > >Yes, I agree Bob, Karel, NL
> > >
> > >Mike Borgelt wrote:
> > >> Water vapour has a molecular weight of a bit over 18 and dry air a
bit
> > >> > more than 28. Water vapour at the same pressure as the air around
it
> > >> > is considerably less dense than dry air. More water vapour= more
> > >> > bouyancy.
> > >
> > >Just a simple approach with rough figures to support Mike's statement
and
> > >hopefully to trigger the "smart guys".
> > >At atmospheric pressure (say 1013 hPa) and at 20 C the density of dry
air
> > >is about 1.22 kg/m3. Pure water vapor at atmospheric pressure has a
> density
> > >of 18/28 x 1.22 = 0.785 kg/m3, or 785 g/m3.
> > >Air is saturated with water vapor when it contains 25 g/m3 at 20 C°.
> > >Assume a relative humidity of say 30% on a dry day. Then one cubic
meter
> of
> > >air contains 0.3 x 25 = 7.5 g of water vapor and the air has then a
> density
> > >of 1.2159 kg/m3. Assume further that over a shallow pond the humidity
of
> the
> > >air increases to 60% due to a serious evaporation from the pond. Then
the
> > >air directly over the pond will contain 0.6 x 25 = 15.0 g/m3
> corresponding
> > >to an air density of 1.2118 kg/m3.
> > >So one cubic meter of air having 60% humidity is 1.2159 - 1.2118=
0.0041
> kg
> > >lighter then air with a humidity of 30%. This 4.1 g/m3 does not look
> much,
> > >but compare this figure with the decrease in density when air is heated
> up.
> > >The temperature coëfficiënt of air is 0.0044 kg/m3 per °C at 20 °C,
> meaning
> > >that when air is heated up by one degree its density decreases with 4.4
> > >g/m3.
> > >So one may conclude that changing the relative humidity of air from 30%
> to
> > >60% has the same effect on buoyancy as raising the temperature of air
by
> 1
> > >°C.
> > >So it may be worthwhile indeed to search for a thermal over a shallow
> pond
> > >in a dry area when low as I stated earlier.
> > >
> > >Karel, NL
> > >
> > But wouldn't the latent heat of evaporation cool the air more that the
> > 1deg C?. In which case a pond wouldn't work. But WTHDIK?
> >
> > About 15 miles east of our site there is a low-lying marshland area
> > about 40 miles across which is all cut up with rivers and drainage
> > canals. I remember reading in an soaring text of the 1970s (I think it
> > was New Soaring Pilot) that it was a good idea to avoid this area
> > because all that water would stop convection.
> >
> > So I asked on of the most experienced club members about it; he said
> > he'd not had any difficulty finding thermals there. He should know, he
> > had several UK records.
>
> > --
> > Mike Lindsay
>
> This supports something I saw back in the 1960's from instrumented
airplane
> traverses made at mid levels in strongly convective conditions . The data
> clearly showed the updrafts corresponding to thermals but did not show any
> temperature rise in the thermals. Instead, they showed an increase in
> absolute humidity corresponding to an increase of about 30% in relative
> humidity over the surrounding air.
>
> It's easy to see that the relative humidity in a thermal steadily
increases
> with height above the ground until it reaches 100% at cloud base. The
> source of most of this moisture has to be the earth's surface below so the
> thermal is a transport mechanism that lifts water vapor up to cloud base.
>
> This has always led me to think that people looking to invent remote
thermal
> sensors should not be looking for water vapor and not warm air. Water
vapor
> has interesting infrared absorption spectra that might allow IR Imaging of
> thermals (it sure works well in weather satellite images). Wingtip
mounted
> wet bulb sensors would directly read the temperature + humidity which
should
> correspond nicely to the spanwise buoyancy gradient and should be a
reliable
> indicator of the best direction of turn when entering a thermal.
>
> Bill Daniels
>

[goneill]

We have a large swamp we usually have to cross each way when xcountry
and I think of as a heat sink while it is at a lower temp than ambient and a
heat radiater (thermals) when day is cooling down in afternoon
gary
"K.P. Termaat" > wrote in message
.. .
> Hi Mike,
>
> The latent heat of evaporation comes to the account of the water in the
> pool. So no problem here.
> Looking for thermals in a marshland with quite some water in it is not
such
> a good idea on blue days. The layer of air close to the ground just does
not
> heat up enough to become unstable looks like. However with some "hot
spots"
> and unstable meteo conditions there need not be a problem in forming
> thermals over marshland, but usually these areas should better be avoided
is
> also my experience.
>
> Karel, NL
>
> "Mike Lindsay" > schreef in bericht
> ...
> > In article >, K.P. Termaat
> > > writes
> > >
> > >"Bob Salvo" > schreef in bericht
> > ...
> > >> Warm breeze picks up moisture at upwing edge of pond. Warm moist air
> > >being
> > >> lighter than dry warm air, begins to rise, initiating thermal.
> > >>
> > >> Happy New Year!
> > >> Bob
> > >
> > >Yes, I agree Bob, Karel, NL
> > >
> > >Mike Borgelt wrote:
> > >> Water vapour has a molecular weight of a bit over 18 and dry air a
bit
> > >> > more than 28. Water vapour at the same pressure as the air around
it
> > >> > is considerably less dense than dry air. More water vapour= more
> > >> > bouyancy.
> > >
> > >Just a simple approach with rough figures to support Mike's statement
and
> > >hopefully to trigger the "smart guys".
> > >At atmospheric pressure (say 1013 hPa) and at 20 C the density of dry
air
> > >is about 1.22 kg/m3. Pure water vapor at atmospheric pressure has a
> density
> > >of 18/28 x 1.22 = 0.785 kg/m3, or 785 g/m3.
> > >Air is saturated with water vapor when it contains 25 g/m3 at 20 C°.
> > >Assume a relative humidity of say 30% on a dry day. Then one cubic
meter
> of
> > >air contains 0.3 x 25 = 7.5 g of water vapor and the air has then a
> density
> > >of 1.2159 kg/m3. Assume further that over a shallow pond the humidity
of
> the
> > >air increases to 60% due to a serious evaporation from the pond. Then
the
> > >air directly over the pond will contain 0.6 x 25 = 15.0 g/m3
> corresponding
> > >to an air density of 1.2118 kg/m3.
> > >So one cubic meter of air having 60% humidity is 1.2159 - 1.2118=
0.0041
> kg
> > >lighter then air with a humidity of 30%. This 4.1 g/m3 does not look
> much,
> > >but compare this figure with the decrease in density when air is heated
> up.
> > >The temperature coëfficiënt of air is 0.0044 kg/m3 per °C at 20 °C,
> meaning
> > >that when air is heated up by one degree its density decreases with 4.4
> > >g/m3.
> > >So one may conclude that changing the relative humidity of air from 30%
> to
> > >60% has the same effect on buoyancy as raising the temperature of air
by
> 1
> > >°C.
> > >So it may be worthwhile indeed to search for a thermal over a shallow
> pond
> > >in a dry area when low as I stated earlier.
> > >
> > >Karel, NL
> > >
> > But wouldn't the latent heat of evaporation cool the air more that the
> > 1deg C?. In which case a pond wouldn't work. But WTHDIK?
> >
> > About 15 miles east of our site there is a low-lying marshland area
> > about 40 miles across which is all cut up with rivers and drainage
> > canals. I remember reading in an soaring text of the 1970s (I think it
> > was New Soaring Pilot) that it was a good idea to avoid this area
> > because all that water would stop convection.
> >
> > So I asked on of the most experienced club members about it; he said
> > he'd not had any difficulty finding thermals there. He should know, he
> > had several UK records.
> >
> > >
> > >
> > >
> > >
> >
> > --
> > Mike Lindsay
>
>

[K.P. Termaat]

I think your way of thinking is correct. I have an equivalent experience for
wooded areas. During the day they do not work well, but at dawn when the
environment is cooling down they give off their accumulated heat and produce
thermals.

Karel, NL


"goneill" > schreef in bericht
...
> We have a large swamp we usually have to cross each way when xcountry
> and I think of as a heat sink while it is at a lower temp than ambient and
a
> heat radiater (thermals) when day is cooling down in afternoon
> gary
> "K.P. Termaat" > wrote in message
> .. .
> > Hi Mike,
> >
> > The latent heat of evaporation comes to the account of the water in the
> > pool. So no problem here.
> > Looking for thermals in a marshland with quite some water in it is not
> such
> > a good idea on blue days. The layer of air close to the ground just does
> not
> > heat up enough to become unstable looks like. However with some "hot
> spots"
> > and unstable meteo conditions there need not be a problem in forming
> > thermals over marshland, but usually these areas should better be
avoided
> is
> > also my experience.
> >
> > Karel, NL
> >
> > "Mike Lindsay" > schreef in bericht
> > ...
> > > In article >, K.P. Termaat
> > > > writes
> > > >
> > > >"Bob Salvo" > schreef in bericht
> > > ...
> > > >> Warm breeze picks up moisture at upwing edge of pond. Warm moist
air
> > > >being
> > > >> lighter than dry warm air, begins to rise, initiating thermal.
> > > >>
> > > >> Happy New Year!
> > > >> Bob
> > > >
> > > >Yes, I agree Bob, Karel, NL
> > > >
> > > >Mike Borgelt wrote:
> > > >> Water vapour has a molecular weight of a bit over 18 and dry air a
> bit
> > > >> > more than 28. Water vapour at the same pressure as the air around
> it
> > > >> > is considerably less dense than dry air. More water vapour= more
> > > >> > bouyancy.
> > > >
> > > >Just a simple approach with rough figures to support Mike's statement
> and
> > > >hopefully to trigger the "smart guys".
> > > >At atmospheric pressure (say 1013 hPa) and at 20 C the density of dry
> air
> > > >is about 1.22 kg/m3. Pure water vapor at atmospheric pressure has a
> > density
> > > >of 18/28 x 1.22 = 0.785 kg/m3, or 785 g/m3.
> > > >Air is saturated with water vapor when it contains 25 g/m3 at 20 C°.
> > > >Assume a relative humidity of say 30% on a dry day. Then one cubic
> meter
> > of
> > > >air contains 0.3 x 25 = 7.5 g of water vapor and the air has then a
> > density
> > > >of 1.2159 kg/m3. Assume further that over a shallow pond the humidity
> of
> > the
> > > >air increases to 60% due to a serious evaporation from the pond. Then
> the
> > > >air directly over the pond will contain 0.6 x 25 = 15.0 g/m3
> > corresponding
> > > >to an air density of 1.2118 kg/m3.
> > > >So one cubic meter of air having 60% humidity is 1.2159 - 1.2118=
> 0.0041
> > kg
> > > >lighter then air with a humidity of 30%. This 4.1 g/m3 does not look
> > much,
> > > >but compare this figure with the decrease in density when air is
heated
> > up.
> > > >The temperature coëfficiënt of air is 0.0044 kg/m3 per °C at 20 °C,
> > meaning
> > > >that when air is heated up by one degree its density decreases with
4.4
> > > >g/m3.
> > > >So one may conclude that changing the relative humidity of air from
30%
> > to
> > > >60% has the same effect on buoyancy as raising the temperature of
air
> by
> > 1
> > > >°C.
> > > >So it may be worthwhile indeed to search for a thermal over a shallow
> > pond
> > > >in a dry area when low as I stated earlier.
> > > >
> > > >Karel, NL
> > > >
> > > But wouldn't the latent heat of evaporation cool the air more that the
> > > 1deg C?. In which case a pond wouldn't work. But WTHDIK?
> > >
> > > About 15 miles east of our site there is a low-lying marshland area
> > > about 40 miles across which is all cut up with rivers and drainage
> > > canals. I remember reading in an soaring text of the 1970s (I think it
> > > was New Soaring Pilot) that it was a good idea to avoid this area
> > > because all that water would stop convection.
> > >
> > > So I asked on of the most experienced club members about it; he said
> > > he'd not had any difficulty finding thermals there. He should know, he
> > > had several UK records.
> > >
> > > >
> > > >
> > > >
> > > >
> > >
> > > --
> > > Mike Lindsay
> >
> >
>
>

[Roger Worden]

In the Jan. 2004 issue of Model Aviation, in the Radio Control Soaring
column, Real Smart Guy candidate Mike Garton proposes a "condensation
analogy" to suggest places to look for thermals to trigger. Imagine water
condensing on a ceiling: it drips first from the low spots or tiny bumps.
Now imagine heated, but relatively stable, air along the ground. If it's
"trying" to rise, might it not "drip up" first from the higher spots, little
hills, even trees? If it's moving slowly horizontally, and encounters a tree
line, it might be forced up enough to trigger a thermal. His experience with
models supports the theory on the small scale. Does y'all's experience
support it at the larger scale?

Roger Worden
>
> Same thing with sandy areas - the books say to avoid them like the
> plague, but the sandy washes here in Arizona are also consistent
> thermal sources - and like the ponds/tanks, are low discontinuities in
> the local terrain. In this case, I'm sure it's not the sand that is
> causing the thermal, my uneducated guess is that the wash channels (or
> collects) the incipient themal until it gets big and strong enough to
> break loose.
>
> Any Real Smart Guys out there care to give us a serious possible
> explanation for these effects? - or maybe we need to keep this to
> ourselves and let the youngsters figure it out for themselves! Got to
> keep a few tricks in our bags, you know, something about age and
> experience beating youth and skill...
>
> Kirk

[goneill]

What you are sying is just about what Reichman says about low level
thermal activity.
I did some exercises in a Ka6 (good short landing glider)
having a ceiling of 1300ft (I left on track at 1300ft) from takeoff point
and
over several flights did a 50km ,65kms ,and 102 kms triangles .
I was prepared to land out but never had to.
Sometimes I would have to drift off track < = 10 kms but I kept to the plan.
I spent a lot of time between 300-800ft agl but still made good times for
that
type of soaring. (When I was competing in later years it was a good way to
burn off leeching pilots) It did no harm to my placings as I got 2 day
placings
of 3rd and finished 5th overall in that nationals.
It was a fantastic learning experience..
If your terrain is suitable and you are confident of your ability to short
land
the glider you should do this exercise ,you will learn a lot.
gary



"Roger Worden" > wrote in message
m...
> In the Jan. 2004 issue of Model Aviation, in the Radio Control Soaring
> column, Real Smart Guy candidate Mike Garton proposes a "condensation
> analogy" to suggest places to look for thermals to trigger. Imagine water
> condensing on a ceiling: it drips first from the low spots or tiny bumps.
> Now imagine heated, but relatively stable, air along the ground. If it's
> "trying" to rise, might it not "drip up" first from the higher spots,
little
> hills, even trees? If it's moving slowly horizontally, and encounters a
tree
> line, it might be forced up enough to trigger a thermal. His experience
with
> models supports the theory on the small scale. Does y'all's experience
> support it at the larger scale?
>
> Roger Worden
> >
> > Same thing with sandy areas - the books say to avoid them like the
> > plague, but the sandy washes here in Arizona are also consistent
> > thermal sources - and like the ponds/tanks, are low discontinuities in
> > the local terrain. In this case, I'm sure it's not the sand that is
> > causing the thermal, my uneducated guess is that the wash channels (or
> > collects) the incipient themal until it gets big and strong enough to
> > break loose.
> >
> > Any Real Smart Guys out there care to give us a serious possible
> > explanation for these effects? - or maybe we need to keep this to
> > ourselves and let the youngsters figure it out for themselves! Got to
> > keep a few tricks in our bags, you know, something about age and
> > experience beating youth and skill...
> >
> > Kirk
>
>

[tango4]

Just about 100%. I don't think this is so much a theory as an analogy.

Ian


"Roger Worden" > wrote in message
m...
> In the Jan. 2004 issue of Model Aviation, in the Radio Control Soaring
> column, Real Smart Guy candidate Mike Garton proposes a "condensation
> analogy" to suggest places to look for thermals to trigger. Imagine water
> condensing on a ceiling: it drips first from the low spots or tiny bumps.
> Now imagine heated, but relatively stable, air along the ground. If it's
> "trying" to rise, might it not "drip up" first from the higher spots,
little
> hills, even trees? If it's moving slowly horizontally, and encounters a
tree
> line, it might be forced up enough to trigger a thermal. His experience
with
> models supports the theory on the small scale. Does y'all's experience
> support it at the larger scale?
>
> Roger Worden
> >
> > Same thing with sandy areas - the books say to avoid them like the
> > plague, but the sandy washes here in Arizona are also consistent
> > thermal sources - and like the ponds/tanks, are low discontinuities in
> > the local terrain. In this case, I'm sure it's not the sand that is
> > causing the thermal, my uneducated guess is that the wash channels (or
> > collects) the incipient themal until it gets big and strong enough to
> > break loose.
> >
> > Any Real Smart Guys out there care to give us a serious possible
> > explanation for these effects? - or maybe we need to keep this to
> > ourselves and let the youngsters figure it out for themselves! Got to
> > keep a few tricks in our bags, you know, something about age and
> > experience beating youth and skill...
> >
> > Kirk
>
>

[Andy Durbin]

"K.P. Termaat" > wrote in message >...
> I think your way of thinking is correct. I have an equivalent experience for
> wooded areas. During the day they do not work well, but at dawn when the
> environment is cooling down they give off their accumulated heat and produce
> thermals.
>
> Karel, NL
>

I expect 99% percent of those reading this will realize *dawn* was
probably a translation error, but just in case, I think you meant
*dusk* or evening.


Andy (GY)

[Gary Evans]

From my experience it appears possible for thermals
to form over and rise from a body of water. In the
past I have done a great deal of hang glider ridge
soaring on the Eastern edge of Lake Michigan. The ridge
is a steep sand bluff about 300-400 feet high right
on the edge of the water. The water is so close that
at some points it touches the base of the bluff. The
lake at this point is about 80 miles wide. On rare
occasions during light winds just capable of sustaining
flight we would encounter what were believed to be
thermals. The water would be almost perfectly flat
and you could see circular disturbances on the surface
moving towards shore at wind speed. When they hit the
bluff you could circle up and drift back over the ridge.
Our only explanation was clouds that caused uneven
heating on the surface of the water were creating the
thermals.

[Andy Durbin]

"Roger Worden" > wrote in message >...
> In the Jan. 2004 issue of Model Aviation, in the Radio Control Soaring
> column, Real Smart Guy candidate Mike Garton proposes a "condensation
> analogy" to suggest places to look for thermals to trigger. Imagine water
> condensing on a ceiling: it drips first from the low spots or tiny bumps.
> Now imagine heated, but relatively stable, air along the ground. If it's
> "trying" to rise, might it not "drip up" first from the higher spots, little
> hills, even trees? If it's moving slowly horizontally, and encounters a tree
> line, it might be forced up enough to trigger a thermal. His experience with
> models supports the theory on the small scale. Does y'all's experience
> support it at the larger scale?
>
> Roger Worden
>

Yes it seems to work that way. A moving object may also disturb
motionless hot air and start a thermal. I was once low over a local
dirt strip, I think turning base to land, when a truck drove into a
large flat dirt area. It triggered a good thermal that got me up and
home.


Andy (GY)

[Bill Daniels]

"Andy Durbin" > wrote in message
om...
> "Roger Worden" > wrote in message
>...
> > In the Jan. 2004 issue of Model Aviation, in the Radio Control Soaring
> > column, Real Smart Guy candidate Mike Garton proposes a "condensation
> > analogy" to suggest places to look for thermals to trigger. Imagine
water
> > condensing on a ceiling: it drips first from the low spots or tiny
bumps.
> > Now imagine heated, but relatively stable, air along the ground. If it's
> > "trying" to rise, might it not "drip up" first from the higher spots,
little
> > hills, even trees? If it's moving slowly horizontally, and encounters a
tree
> > line, it might be forced up enough to trigger a thermal. His experience
with
> > models supports the theory on the small scale. Does y'all's experience
> > support it at the larger scale?
> >
> > Roger Worden
> >
>
> Yes it seems to work that way. A moving object may also disturb
> motionless hot air and start a thermal. I was once low over a local
> dirt strip, I think turning base to land, when a truck drove into a
> large flat dirt area. It triggered a good thermal that got me up and
> home.
> > Andy (GY)

One thing to keep in mind is that there is a ratio between thermal triggers
and the heated air available to be triggered.

In other words, in weak conditions over rugged terrain, there is a surplus
of available triggers, but a deficit of heater air to be triggered.
Sometimes there will be no thermal over an obvious trigger site because the
available bouyant air was already triggered by a lesser, but adequate
trigger upwind. In these cases, potential trigger sites are not a reliable
thermal indicator.

In strong conditions, over mostly uniform, flat surfaces, the few available
trigger sites become more important.

Bill Daniels

[K.P. Termaat]

Hi Andy,

Thanks for your correction. Meant "dusk" i.s.o. "dawn". Translation error
indeed. Though because of my work (american) english is about my second
language I made this slip of the pen.

Karel, NL


"Andy Durbin" > schreef in bericht
om...
> "K.P. Termaat" > wrote in message
>...
> > I think your way of thinking is correct. I have an equivalent experience
for
> > wooded areas. During the day they do not work well, but at dawn when the
> > environment is cooling down they give off their accumulated heat and
produce
> > thermals.
> >
> > Karel, NL
> >
>
> I expect 99% percent of those reading this will realize *dawn* was
> probably a translation error, but just in case, I think you meant
> *dusk* or evening.
>
>
> Andy (GY)

[K.P. Termaat]

Yes I agree. Another example is big cities versus small ones.
I almost never find thermals over the larger areas of housings and buildings
belonging to a somewhat larger urban area. Just to many trigger points
producing small and low thermals only. However when over a village or a
group of farm housings it is usually very easy to find the spot where good
thermals are triggered off using the heated air of the direct environment.

Karel, NL


"Bill Daniels" > schreef in bericht
hlink.net...
>
> "Andy Durbin" > wrote in message
> om...
> > "Roger Worden" > wrote in message
> >...
> > > In the Jan. 2004 issue of Model Aviation, in the Radio Control Soaring
> > > column, Real Smart Guy candidate Mike Garton proposes a "condensation
> > > analogy" to suggest places to look for thermals to trigger. Imagine
> water
> > > condensing on a ceiling: it drips first from the low spots or tiny
> bumps.
> > > Now imagine heated, but relatively stable, air along the ground. If
it's
> > > "trying" to rise, might it not "drip up" first from the higher spots,
> little
> > > hills, even trees? If it's moving slowly horizontally, and encounters
a
> tree
> > > line, it might be forced up enough to trigger a thermal. His
experience
> with
> > > models supports the theory on the small scale. Does y'all's experience
> > > support it at the larger scale?
> > >
> > > Roger Worden
> > >
> >
> > Yes it seems to work that way. A moving object may also disturb
> > motionless hot air and start a thermal. I was once low over a local
> > dirt strip, I think turning base to land, when a truck drove into a
> > large flat dirt area. It triggered a good thermal that got me up and
> > home.
> > > Andy (GY)
>
> One thing to keep in mind is that there is a ratio between thermal
triggers
> and the heated air available to be triggered.
>
> In other words, in weak conditions over rugged terrain, there is a surplus
> of available triggers, but a deficit of heater air to be triggered.
> Sometimes there will be no thermal over an obvious trigger site because
the
> available bouyant air was already triggered by a lesser, but adequate
> trigger upwind. In these cases, potential trigger sites are not a
reliable
> thermal indicator.
>
> In strong conditions, over mostly uniform, flat surfaces, the few
available
> trigger sites become more important.
>
> Bill Daniels
>

[John Galloway]

As regards cities I think it depends...

I once went on a lead and follow course with Andy Davis
in blue or partially blue weather. In briefing before
flying he would nominate where he was planning to take
climbs. In the case of larger towns and cities (Bath,
Worcester and Swindon come particularly to mind) he
specified where the best thermal source would be and
would then take us there as low as possible. If a
strong core wasn't there when we arrived he parked
us in weak lift and nosed around until he found the
next strong pulse. It seemed odd at first to take
so much time to do that but it was worth it because
he had saved so much more time by ignoring weak lift
in the cruise to reach his nominated best thermal source
with the minimum of delay and at an altitude low enough
to take best advantage of the good climb.

So I think that cities will be likely to have at least
one source that is better than the multitude of little
trigger points

John Galloway

At 21:24 04 January 2004, K.P. Termaat wrote:
>Yes I agree. Another example is big cities versus small
>ones.
>I almost never find thermals over the larger areas
>of housings and buildings
>belonging to a somewhat larger urban area. Just to
>many trigger points
>producing small and low thermals only. However when
>over a village or a
>group of farm housings it is usually very easy to find
>the spot where good
>thermals are triggered off using the heated air of
>the direct environment.
>
>Karel, NL
>
>
>'Bill Daniels' schreef in bericht
hlink.net...
>>
>> 'Andy Durbin' wrote in message
>> om...
>> > 'Roger Worden' wrote in message
>> news:...
>> > > In the Jan. 2004 issue of Model Aviation, in the
>>>>Radio Control Soaring
>> > > column, Real Smart Guy candidate Mike Garton proposes
>>>>a 'condensation
>> > > analogy' to suggest places to look for thermals
>>>>to trigger. Imagine
>> water
>> > > condensing on a ceiling: it drips first from the
>>>>low spots or tiny
>> bumps.
>> > > Now imagine heated, but relatively stable, air along
>>>>the ground. If
>it's
>> > > 'trying' to rise, might it not 'drip up' first from
>>>>the higher spots,
>> little
>> > > hills, even trees? If it's moving slowly horizontally,
>>>>and encounters
>a
>> tree
>> > > line, it might be forced up enough to trigger a
>>>>thermal. His
>experience
>> with
>> > > models supports the theory on the small scale. Does
>>>>y'all's experience
>> > > support it at the larger scale?
>> > >
>> > > Roger Worden
>> > >
>> >
>> > Yes it seems to work that way. A moving object may
>>>also disturb
>> > motionless hot air and start a thermal. I was once
>>>low over a local
>> > dirt strip, I think turning base to land, when a
>>>truck drove into a
>> > large flat dirt area. It triggered a good thermal
>>>that got me up and
>> > home.
>> > > Andy (GY)
>>
>> One thing to keep in mind is that there is a ratio
>>between thermal
>triggers
>> and the heated air available to be triggered.
>>
>> In other words, in weak conditions over rugged terrain,
>>there is a surplus
>> of available triggers, but a deficit of heater air
>>to be triggered.
>> Sometimes there will be no thermal over an obvious
>>trigger site because
>the
>> available bouyant air was already triggered by a lesser,
>>but adequate
>> trigger upwind. In these cases, potential trigger
>>sites are not a
>reliable
>> thermal indicator.
>>
>> In strong conditions, over mostly uniform, flat surfaces,
>>the few
>available
>> trigger sites become more important.
>>
>> Bill Daniels
>>
>
>
>

[Aspley Nursery]

As a horticulturalist and a glider pilot, perhaps the answer is that the
moist ground acts as more of a heat sink into the ground, up until it
achieves a full heat load and then dissipates this in the afternoon, more so
than the surrounding dry ground. Vegetation on a west facing slope is a
great source in the afternoons out our way.

I have found lift in moister areas, but much more so in the afternoons,
mainly when lower in the convection zone and certainly not in the mornings.
Try walking around a wet area at 10am and 6pm and compare the relative heat
to dry areas.

The other issue that could be occurring is the bubble of cool air over the
moist ground could be acting as a trigger point, for the drifting heated air
from the surrounding dry areas. We all know of the tremendously small things
that can act as a trigger.

As to lapse rates the air is not saturated till cloud base, unless of course
a fog is present (mornings) which is a different case than trying to get a
thermal.

Robert P
Nimbus 2C

"Mike Rapoport" > wrote in message
hlink.net...
>
> "Andrew Sarangan" > wrote in message
> om...
> > (Kirk Stant) wrote in message
> >...
> > > "K.P. Termaat" > wrote in message
> >...
> > > > My experience is that it works, especially on days with very low
> humidity,
> > > > but no boomers and only low.
> > > >
> > > >
> > > > "Mike Rapoport" > schreef in bericht
> > > > ink.net...
> > > > >
> > > > > You will find less lift over water of any kind, even if it is
> contained in
> > > > > vegetation. The best lift is always over the highest, dryest,
> darkest
> > > > > surface around. The water vapor idea is...well...it is hard to
find
> a
> > > place
> > > > > to start...but it won't work
> > > > >
> > > > > Mike
> > > > > MU-2
> > > > >
> > >
> > > Have to disagree with you, Mike - out here in Arizona, in the desert
> > > areas that are not irrigated, we often find good lift directly over
> > > small cattle "tanks" - small shallow ponds that are scattered around.
> > > A lot of us have noticed this and compared notes, and it works; if too
> > > low to get to high, dark ground, I'll head for the nearest pond and it
> > > will usually turn up a nice thermal. We think it may be due to the
> > > fact that the ponds are in a natural low spot, and coupled with the
> > > little bit of moisture, could be the necessary trigger for a thermal.
> > >
> > > Now obviously, large irrigated farm fields or river basins are death
> > > to thermals - but a local lake (reservoir) seems to have little effect
> > > on thermal activity - could it be all the drunk boaters?
> > >
> > > What's the old saying about never saying never?
> > >
> > > Kirk
> > > LS6-b
> >
> >
> > The adiabatic rate of moist air is about 1.5C/1000ft. For dry air it
> > is 3C/1000ft. Therefore, assuming that the water temperature is the
> > same as the surrounding ground (which would be true if the water is
> > shallow), I could see how one will experience greater lift above
> > water.
>
> Not for unsaturated air it isn't. The moist rate only applies to saturated
> air (ie in clouds)
>
> Mike
> MU-2
>
>

[goneill]

"goneill" > wrote in message
...
>
> "Aspley Nursery" > wrote in message
> ...
> Robert P
> Nimbus 2C
>
> I see in your sig Nimbus 2 C
> That model is one I have been considering for some time.
> Do you have any comments on it in comparison to other open class
> gliders and in particular its approaches with the ventus style brakes.
> Because there are none here I have some contrary comments from
> other club members and even considered just getting a ASW20
> instead .
> The main reason I have been looking at it is most of the better pilots
> are flying ventus's 18m but my budget only runs to an older open class
> How does the 2c keep up with them all be it with a bit of water on to
> get the wing loading up.The main "anti" comment is that it won't keep
> up with its thicker aerofoil section and thus negating the main reason
> why I am considering one.
> gary
>
>
>
>

[Andrew Sarangan]

"Mike Rapoport" > wrote in message .net>...
> "Andrew Sarangan" > wrote in message
> om...
> > (Kirk Stant) wrote in message
> >...
> > > "K.P. Termaat" > wrote in message
> >...
> > > > My experience is that it works, especially on days with very low
> humidity,
> > > > but no boomers and only low.
> > > >
> > > >
> > > > "Mike Rapoport" > schreef in bericht
> > > > ink.net...
> > > > >
> > > > > You will find less lift over water of any kind, even if it is
> contained in
> > > > > vegetation. The best lift is always over the highest, dryest,
> darkest
> > > > > surface around. The water vapor idea is...well...it is hard to find
> a
> place
> > > > > to start...but it won't work
> > > > >
> > > > > Mike
> > > > > MU-2
> > > > >
> > >
> > > Have to disagree with you, Mike - out here in Arizona, in the desert
> > > areas that are not irrigated, we often find good lift directly over
> > > small cattle "tanks" - small shallow ponds that are scattered around.
> > > A lot of us have noticed this and compared notes, and it works; if too
> > > low to get to high, dark ground, I'll head for the nearest pond and it
> > > will usually turn up a nice thermal. We think it may be due to the
> > > fact that the ponds are in a natural low spot, and coupled with the
> > > little bit of moisture, could be the necessary trigger for a thermal.
> > >
> > > Now obviously, large irrigated farm fields or river basins are death
> > > to thermals - but a local lake (reservoir) seems to have little effect
> > > on thermal activity - could it be all the drunk boaters?
> > >
> > > What's the old saying about never saying never?
> > >
> > > Kirk
> > > LS6-b
> >
> >
> > The adiabatic rate of moist air is about 1.5C/1000ft. For dry air it
> > is 3C/1000ft. Therefore, assuming that the water temperature is the
> > same as the surrounding ground (which would be true if the water is
> > shallow), I could see how one will experience greater lift above
> > water.
>
> Not for unsaturated air it isn't. The moist rate only applies to saturated
> air (ie in clouds)
>
> Mike
> MU-2


OK, 1.5C/1000ft applies only to 100% RH air. But a 50% RH air must
still have a lower lapse rate than dry air, no?

[K.P. Termaat]

Andrew, read the numerical support of Mike Borgelt's statement below which I
posted a few days ago.


Mike Borgelt wrote:
> Water vapour has a molecular weight of a bit over 18 and dry air a bit
> > more than 28. Water vapour at the same pressure as the air around it
> > is considerably less dense than dry air. More water vapour= more
> > bouyancy.

Just a simple approach with rough figures to support Mike's statement and
hopefully to trigger the "smart guys".
At atmospheric pressure (say 1013 hPa) and at 20 C° the density of dry air
is about 1.22 kg/m3. Pure water vapor at atmospheric pressure has a density
of 18/28 x 1.22 = 0.785 kg/m3, or 785 g/m3.
Air is saturated with water vapor when it contains 25 g/m3 at 20 C°.
Assume a relative humidity of say 30% on a dry day. Then one cubic meter of
air contains 0.3 x 25 = 7.5 g of water vapor and the air has then a density
of 1.2159 kg/m3. Assume further that over a shallow pond the humidity of the
air increases to 60% due to a serious evaporation from the pond. Then the
air directly over the pond will contain 0.6 x 25 = 15.0 g/m3 corresponding
to an air density of 1.2118 kg/m3.
So one cubic meter of air having 60% humidity is 1.2159 - 1.2118= 0.0041 kg
lighter then air with a humidity of 30%. This 4.1 g/m3 does not look much,
but compare this figure with the decrease in density when air is heated up.
The temperature coëfficiënt of air is 0.0044 kg/m3 per °C at 20 °C, meaning
that when air is heated up by one degree its density decreases with 4.4
g/m3.
So one may conclude that changing the relative humidity of air from 30% to
60% has the same effect on buoyancy as raising the temperature of air by 1
°C.
So it may be worthwhile indeed to search for a thermal over a shallow pond
in a dry area when low as I stated earlier.

Karel, NL


"Andrew Sarangan" > schreef in bericht
om...
> "Mike Rapoport" > wrote in message
.net>...
> > "Andrew Sarangan" > wrote in message
> > om...
> > > (Kirk Stant) wrote in message
> > >...
> > > > "K.P. Termaat" > wrote in message
> > >...
> > > > > My experience is that it works, especially on days with very low
> > humidity,
> > > > > but no boomers and only low.
> > > > >
> > > > >
> > > > > "Mike Rapoport" > schreef in bericht
> > > > > ink.net...
> > > > > >
> > > > > > You will find less lift over water of any kind, even if it is
> > contained in
> > > > > > vegetation. The best lift is always over the highest, dryest,
> > darkest
> > > > > > surface around. The water vapor idea is...well...it is hard to
find
> > a
> > place
> > > > > > to start...but it won't work
> > > > > >
> > > > > > Mike
> > > > > > MU-2
> > > > > >
> > > >
> > > > Have to disagree with you, Mike - out here in Arizona, in the desert
> > > > areas that are not irrigated, we often find good lift directly over
> > > > small cattle "tanks" - small shallow ponds that are scattered
around.
> > > > A lot of us have noticed this and compared notes, and it works; if
too
> > > > low to get to high, dark ground, I'll head for the nearest pond and
it
> > > > will usually turn up a nice thermal. We think it may be due to the
> > > > fact that the ponds are in a natural low spot, and coupled with the
> > > > little bit of moisture, could be the necessary trigger for a
thermal.
> > > >
> > > > Now obviously, large irrigated farm fields or river basins are death
> > > > to thermals - but a local lake (reservoir) seems to have little
effect
> > > > on thermal activity - could it be all the drunk boaters?
> > > >
> > > > What's the old saying about never saying never?
> > > >
> > > > Kirk
> > > > LS6-b
> > >
> > >
> > > The adiabatic rate of moist air is about 1.5C/1000ft. For dry air it
> > > is 3C/1000ft. Therefore, assuming that the water temperature is the
> > > same as the surrounding ground (which would be true if the water is
> > > shallow), I could see how one will experience greater lift above
> > > water.
> >
> > Not for unsaturated air it isn't. The moist rate only applies to
saturated
> > air (ie in clouds)
> >
> > Mike
> > MU-2
>
>
> OK, 1.5C/1000ft applies only to 100% RH air. But a 50% RH air must
> still have a lower lapse rate than dry air, no?

[Derrick Steed]

If that were true, you would also find a VERY low cloud over the same spot.

Rgds,

Derrick.

[K.P. Termaat]

Due to mixing with dryer air the humidity is spread out after a while and
condensation does not take place.

Karel, NL


"Derrick Steed" > schreef in bericht
...
> If that were true, you would also find a VERY low cloud over the same
spot.
>
> Rgds,
>
> Derrick.
>
>
>

[Mike Rapoport]

"Andrew Sarangan" > wrote in message
om...
> "Mike Rapoport" > wrote in message
.net>...
> > "Andrew Sarangan" > wrote in message
> > om...
> > > (Kirk Stant) wrote in message
> > >...
> > > > "K.P. Termaat" > wrote in message
> > >...
> > > > > My experience is that it works, especially on days with very low
> > humidity,
> > > > > but no boomers and only low.
> > > > >
> > > > >
> > > > > "Mike Rapoport" > schreef in bericht
> > > > > ink.net...
> > > > > >
> > > > > > You will find less lift over water of any kind, even if it is
> > contained in
> > > > > > vegetation. The best lift is always over the highest, dryest,
> > darkest
> > > > > > surface around. The water vapor idea is...well...it is hard to
find
> > a
> > place
> > > > > > to start...but it won't work
> > > > > >
> > > > > > Mike
> > > > > > MU-2
> > > > > >
> > > >
> > > > Have to disagree with you, Mike - out here in Arizona, in the desert
> > > > areas that are not irrigated, we often find good lift directly over
> > > > small cattle "tanks" - small shallow ponds that are scattered
around.
> > > > A lot of us have noticed this and compared notes, and it works; if
too
> > > > low to get to high, dark ground, I'll head for the nearest pond and
it
> > > > will usually turn up a nice thermal. We think it may be due to the
> > > > fact that the ponds are in a natural low spot, and coupled with the
> > > > little bit of moisture, could be the necessary trigger for a
thermal.
> > > >
> > > > Now obviously, large irrigated farm fields or river basins are death
> > > > to thermals - but a local lake (reservoir) seems to have little
effect
> > > > on thermal activity - could it be all the drunk boaters?
> > > >
> > > > What's the old saying about never saying never?
> > > >
> > > > Kirk
> > > > LS6-b
> > >
> > >
> > > The adiabatic rate of moist air is about 1.5C/1000ft. For dry air it
> > > is 3C/1000ft. Therefore, assuming that the water temperature is the
> > > same as the surrounding ground (which would be true if the water is
> > > shallow), I could see how one will experience greater lift above
> > > water.
> >
> > Not for unsaturated air it isn't. The moist rate only applies to
saturated
> > air (ie in clouds)
> >
> > Mike
> > MU-2
>
>
> OK, 1.5C/1000ft applies only to 100% RH air. But a 50% RH air must
> still have a lower lapse rate than dry air, no?

No. The reason that saturated air lapses at a slower rate is that latent
energy is being released as the water vapor changes to liquid, that is the
only reason.

Mike
MU-2

[Mike Rapoport]

I don't disagree with your math but the air over the water is also cooler
than the air over the dry land adjacent to the pond.

Mike
MU-2

"K.P. Termaat" > wrote in message
...
> Andrew, read the numerical support of Mike Borgelt's statement below which
I
> posted a few days ago.
>
>
> Mike Borgelt wrote:
> > Water vapour has a molecular weight of a bit over 18 and dry air a bit
> > > more than 28. Water vapour at the same pressure as the air around it
> > > is considerably less dense than dry air. More water vapour= more
> > > bouyancy.
>
> Just a simple approach with rough figures to support Mike's statement and
> hopefully to trigger the "smart guys".
> At atmospheric pressure (say 1013 hPa) and at 20 C° the density of dry air
> is about 1.22 kg/m3. Pure water vapor at atmospheric pressure has a
density
> of 18/28 x 1.22 = 0.785 kg/m3, or 785 g/m3.
> Air is saturated with water vapor when it contains 25 g/m3 at 20 C°.
> Assume a relative humidity of say 30% on a dry day. Then one cubic meter
of
> air contains 0.3 x 25 = 7.5 g of water vapor and the air has then a
density
> of 1.2159 kg/m3. Assume further that over a shallow pond the humidity of
the
> air increases to 60% due to a serious evaporation from the pond. Then the
> air directly over the pond will contain 0.6 x 25 = 15.0 g/m3 corresponding
> to an air density of 1.2118 kg/m3.
> So one cubic meter of air having 60% humidity is 1.2159 - 1.2118= 0.0041
kg
> lighter then air with a humidity of 30%. This 4.1 g/m3 does not look
much,
> but compare this figure with the decrease in density when air is heated
up.
> The temperature coëfficiënt of air is 0.0044 kg/m3 per °C at 20 °C,
meaning
> that when air is heated up by one degree its density decreases with 4.4
> g/m3.
> So one may conclude that changing the relative humidity of air from 30% to
> 60% has the same effect on buoyancy as raising the temperature of air by
1
> °C.
> So it may be worthwhile indeed to search for a thermal over a shallow pond
> in a dry area when low as I stated earlier.
>
> Karel, NL
>
>
> "Andrew Sarangan" > schreef in bericht
> om...
> > "Mike Rapoport" > wrote in message
> .net>...
> > > "Andrew Sarangan" > wrote in message
> > > om...
> > > > (Kirk Stant) wrote in message
> > > >...
> > > > > "K.P. Termaat" > wrote in message
> > > >...
> > > > > > My experience is that it works, especially on days with very low
> > > humidity,
> > > > > > but no boomers and only low.
> > > > > >
> > > > > >
> > > > > > "Mike Rapoport" > schreef in
bericht
> > > > > > ink.net...
> > > > > > >
> > > > > > > You will find less lift over water of any kind, even if it is
> > > contained in
> > > > > > > vegetation. The best lift is always over the highest, dryest,
> > > darkest
> > > > > > > surface around. The water vapor idea is...well...it is hard
to
> find
> > > a
> > > place
> > > > > > > to start...but it won't work
> > > > > > >
> > > > > > > Mike
> > > > > > > MU-2
> > > > > > >
> > > > >
> > > > > Have to disagree with you, Mike - out here in Arizona, in the
desert
> > > > > areas that are not irrigated, we often find good lift directly
over
> > > > > small cattle "tanks" - small shallow ponds that are scattered
> around.
> > > > > A lot of us have noticed this and compared notes, and it works; if
> too
> > > > > low to get to high, dark ground, I'll head for the nearest pond
and
> it
> > > > > will usually turn up a nice thermal. We think it may be due to
the
> > > > > fact that the ponds are in a natural low spot, and coupled with
the
> > > > > little bit of moisture, could be the necessary trigger for a
> thermal.
> > > > >
> > > > > Now obviously, large irrigated farm fields or river basins are
death
> > > > > to thermals - but a local lake (reservoir) seems to have little
> effect
> > > > > on thermal activity - could it be all the drunk boaters?
> > > > >
> > > > > What's the old saying about never saying never?
> > > > >
> > > > > Kirk
> > > > > LS6-b
> > > >
> > > >
> > > > The adiabatic rate of moist air is about 1.5C/1000ft. For dry air it
> > > > is 3C/1000ft. Therefore, assuming that the water temperature is the
> > > > same as the surrounding ground (which would be true if the water is
> > > > shallow), I could see how one will experience greater lift above
> > > > water.
> > >
> > > Not for unsaturated air it isn't. The moist rate only applies to
> saturated
> > > air (ie in clouds)
> > >
> > > Mike
> > > MU-2
> >
> >
> > OK, 1.5C/1000ft applies only to 100% RH air. But a 50% RH air must
> > still have a lower lapse rate than dry air, no?
>
>

[Derrick Steed]

Reply to KP Termaat "Due to mixing with dryer air the humidity is spread out
after a while and
condensation does not take place".
I don't agree - common models of thermal convection are based on the
assumption that the air parcel (or column for that matter) rises
"adiabatically". If you refer to any text on Thermodynamics you will find
that "adiabatic" means that there is no exchange of energy with the
surroundings, "mixing with drier air" would invalidate that assumption.
My point was that any mixing of water vapour with the air above the pond
would certainly not take the humidity in the parcel as high as 60% except in
a region very close to the surface of the water - this may have something to
do with the "thermal" not sticking to the surface (is there such a thing as
the surface tension of a thermal bubble?) as much as it would over a dry
surface.
Rgds,
Derrick.

[[email protected]]

I've followed most of this thread and have found it interesting. I
think Mike Borgelt's analysis is a good accounting of what's going on.
I get to deal with underground mine ventilation and it has some
interesting analogies to this situation, water vapor and thermals. In
some situations moist air underground will be lighter than air in
other parts of a mine or tunnel and the difference in the "wieght"
will be sufficient to change the air flow - just because of humidity!
Most people, and pilots too - are surprised that dry air is heavier
than air with some moisture in it. A parcel at a given temperature
will be the lightest at 100% RH. I've been in thermals that I'm sure
originated in stock tanks or isolated ponds out west where the air is
"relatively" dry. It doesn't occurr in New England though, I'd guess
because there isn't enough of a humidity differential between the two
air parcels ( over the water, and adjacent ) and thier relative
humidities. Out west where the air is drier this does occurr and could
very well trigger a thermal. And then probably in conditions where
land forms or the area is conducive to preventing mixing with
surounding air by wind - a sheltered area or no air movement to allow
a parcel to build above a water source. And then the bouyant
difference in the air masses due to the humidity diferences has to be
enough to overcome the air cooling near the surface of the water ( the
swamp cooler thing! ) by the evaporation taking place into the air
above the water. Clear as mud - Mark Guay