Any sailplane pilots?

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

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

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

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

"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

"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)

"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

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 . com...
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 hlink.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

"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,

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 hlink.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