Originally Posted by Eric Greenwell

Denis wrote:

And also, why would you get blown out of a thermal? Since you are
circling in it, shouldn't you drift at it's speed, instead of the wind
speed?


THat's perfectly true if the thermal is vertical, which should be the
case if it is not triggered by a ground feature, and with no significant
wind shear. In the other case, as you climb slower than the air in the
thermal, you will get under it and have to correct upwind to find it
again (another solution for us impure pilots is start the engine to
climb again in the original part of the thermal without ajust our
circling ;-) )

How high do you think this effect (the acceleration of the thermal
until it matches the wind speed) persists? I would expect the thermal
drift to match the wind speed in less than 2000 feet agl, based on
observations of dust devils in our area.


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Eric Greenwell
Washington State
USA