bagmaker wrote:
Eric Greenwell Wrote:

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 beleive the windspeed to be changing through height gradient,
sometimes decreasing, usually increasing with altitude, thus a rising
thermal mass will always be chasing the speed of the wind, not meeting
it.

True, but the variations I see are usually small enough, that I think
the thermal quickly adjusts so the difference is always less than 2
knots. I picked 2 knots because that is the kind of difference I
sometimes see in the wind measurements between circling and cruising
flight. But, perhaps there is a way to measure this:

* the pilot leaves the thermal
* he begins to circle again as soon as he is out of the lift
* after about 4 circles, he continues on his flight
* post-flight, the wind drift in the thermal and out of the thermal can
be compared from the flight record

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Eric Greenwell
Washington State
USA