I'm going to retract these comments about the thermal position relative
to the glider. The comments about how often I have to correct upwind are
still right, but that perhaps it's because where I usually fly might
have most of the thermals moving over the ground, rather than
originating at point sources. I don't think the comments apply to the
case Todd described, though they do seem to apply to what I encounter
where I normally fly, and to the shape of some dust devils.

These dust devils are moving over the ground, though more slowly than
the wind aloft. The bottom 1000 feet or so appears to go straight up,
then it bends over quickly until it's about a 10-30 degree angle with
the ground, has a relatively straight portion, then it bends up and is
straight again. I'm not sure of this, but perhaps the straight, bent
over portion is elongating as it speeds up to match the wind speed, then
begins to feed the upper air mass as I described below. The effect would
be a thermal source that travels with the wind, but being fed by a
slower moving source on the ground.

Eric Greenwell wrote:

T o d d P a t t i s t wrote:

Eric Greenwell wrote:


Once the thermal and the airmass are moving at the same speed, there
would be no need to correct upwind.



Imagine a 10 knot thermal being continuously generated from
a quarry or other warm spot on the ground. Assume a 10 knot
steady breeze with no speed change with altitude (no wind
shear). In one tenth of an hour (6 minutes) the thermal
will have risen to one nautical mile high (6,000') and its
top will have drifted one nautical mile downwind of the
quarry.

Now imagine a glider at 6,000' that began to circle (in
sink) directly over the quarry when the thermal started.
The glider has a 2.5 knot descent rate when turning. In the
absence of the thermal, in the same six minutes, the glider
would be circling about 1500' lower and have drifted the
same one nautical mile downwind of the quarry. Clearly, the
descending downwind angled path of the glider (dropping from
6000' to 4500') and the rising downwind track of the thermal
(rising from 0' to 6000') must cross, so what happens at
that point?

The answer is simply that the glider begins to rise as it
descends into the rising air. However, it does not rise as
fast as the thermal. It's still descending at the 2.5 knot
descent rate relative to the rising 10 knot thermal. Each
instant that the glider is in the rising air, it is
descending slightly in the thermal, and each bit of descent
takes it into air that left the ground later and was
slightly farther upwind relative to where the glider
started.


This is where this model is wrong. What you describe is true near the
ground, where the airmass speed exceeds the thermal source (the ground
point) speed by 10 knots. At 1000', the airmass speed is still 10 knots,
but the thermal speed is now (for example) 5 knots because the the wind
has accelerated it; i.e., the airmass above 1000' is being fed by a
_moving_ source, not a stationary one.

At some point (I suggest 2000') the thermal has accelerated to the same
horizontal speed as the air mass. At that point, the airmass above 2000'
is being fed by a thermal source (the airmass at 2000') that is moving
at the same speed it is.

Eventually the glider drops out the bottom of the
angled downwind, rising, path of the thermal (provided the
glider makes no centering corrections) and it continues its
downward and downwind drifting path, having been delayed as
its descending path crossed the thermal's rising path.


As long as the glider enters the thermal above 2000' (in this case), it
will not drop out of thermal, since the thermal is moving at the wind
speed. In fact, this is usually the case I encounter, because most of my
thermals do not require an upwind correction.

So far, no one has commented on my suggestion we measure the difference
in the wind speed and the thermal drift by circling a few times after we
leave a thermal, then comparing the drift from the flight trace later.
Does anyone have a better idea?



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Eric Greenwell
Washington State
USA