Eric Greenwell a écrit :
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.

I don't really know. I found up to 10 kmh difference but it might be measuring errors.

But even if the thermal moves at wind speed, if it is continuously climbing from the same ground point, it will be oblique (downwind from its triggering point) hence the need to correct upwind from time to time...

Denis