Denis wrote:
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...

It would be oblique with respect to the airmass until it had the same
speed; then, it would be vertical. It would always look oblique to
someone looking at the thermal (if it had dust or gliders in it, for
example) while standing on the ground. Once the thermal and the airmass
are moving at the same speed, there would be no need to correct upwind.

What I'm not sure about is how long it takes for the thermal to speed up
to the airmass speed. What do you think of the idea of circling for a
few turns after leaving a thermal? I think this might let us measure the
speed difference, if any, between a thermal and the airmass.


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Eric Greenwell
Washington State
USA