I think this rather depends on the degree of instability in the air. On
relatively stable days the usual thermal sources often don't seem to
work. You need a large area to remain undisturbed for some time until
enough hot air is available to give a decent thermal when triggered. Often
things like towns and motorways don't work because there is too much
trigger activity going on and only tiny weak thermals will form that go to
no great height.

Derek Copeland

At 14:01 23 September 2009, Martin Gregorie wrote:
On Wed, 23 Sep 2009 01:45:02 +0000, Nyal Williams wrote:

I've heard pilots say that the departure of an aircraft will kick off
a
thermal from the runway. Worth watching if you are relatively low
over
the field and your timing is right to search it out.

Its well known in the free flight model flying world that in the right
conditions a few people running about flapping T-shirts can kick off a
thermal. I once made my own thermal just by running about circle towing
an F1A class competition glider in sparse foot-high dry grass on calm
early morning conditions. It wasn't strong though - just enough to make
a
model with a 0.3 m/s min sink speed climb slowly.


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martin@ | Martin Gregorie
gregorie. | Essex, UK
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