My somewhat uneducated guess is that any form of discontinuity is enough to
act as a trigger source for thermals. Doesnt really matter whether its
dark/light, high/low, dry/wet. Anything that breaks up the surface layer of
air warmed by the ground and starts any form of vertical motion will work.

--
Regards,

Adrian Jansen
J & K MicroSystems
Microcomputer solutions for industrial control

"Kirk Stant" wrote in message
om...
Mike Borgelt wrote in message
. ..

Water vapour has a molecular weight of a bit over 18 and dry air a bit
more than 28. Water vapour at the same pressure as the air around it
is considerably less dense than dry air. More water vapour= more
bouyancy.

Then again this may have more to do with low spots in the ground. I've
always found quarries (holes in the ground)to be excellent lift
sources when low.

This discussion is fascinating. I've been flying gliders for some 27
years and have read a lot of books on the theory and practice (Moffat,
Reichmann, Piggott, etc) and never ran into any reference to this
thermal source (or trigger mechanism) - but here we have pilots from
three continents describing apparently the same, common, reliable
trigger mechanism - all apparently discovered empirically (thats how I
found it, that and following Andy around trying to keep up with him
) - Everyone always said head for the dry, high, dark ground, and
here are experienced pilots heading for a low pond!

Same thing with sandy areas - the books say to avoid them like the
plague, but the sandy washes here in Arizona are also consistent
thermal sources - and like the ponds/tanks, are low discontinuities in
the local terrain. In this case, I'm sure it's not the sand that is
causing the thermal, my uneducated guess is that the wash channels (or
collects) the incipient themal until it gets big and strong enough to
break loose.

Any Real Smart Guys out there care to give us a serious possible
explanation for these effects? - or maybe we need to keep this to
ourselves and let the youngsters figure it out for themselves! Got to
keep a few tricks in our bags, you know, something about age and
experience beating youth and skill...

Kirk