Any sailplane pilots?

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.

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

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

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

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

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

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)

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.

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

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 . com...
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