Mike Rapoport wrote:

"K.P. Termaat" wrote in message
...
Thanks Larry for your interesting respons with your links to the articles.


Talking about a drain and water brings me to the idea of telling that when
low and looking for a thermal I always try to locate small ponds in dry
areas. It looks to me that the water vapor rising from these ponds is an
excellent means of starting a thermal. Water vapor is lighter then air, so
it increases the boyancy of the air over the pond and off it goes.
Starting
at about ground level, circling to the right may then generally be the
better option. Any experience with this Larry?

Karel, NL

You will find less lift over water of any kind, even if it is contained in
vegetation. The best lift is always over the highest, dryest, darkest
surface around. The water vapor idea is...well...it is hard to find a place
to start...but it won't work

Mike
MU-2

As I just posted in the "water vapour" thread, I think you have it all back the front.
Have seen similar effects over the small dams on farms here too. My idea is that the air over the
water cools by evaporating water out of the pond. In so doing it looses more heat and hence
contracts more (gets denser) than it gains buoyancy by water vapour increase, ie, it gets both
colder and denser overall than the surrounding surface air. As the dense pool of air becomes
greater, it spreads out, ie, sort of collapses on itself, and pushes out over the edges of the pond
/ dam, particularly down slope over the dam wall, creating a miniature equivalent of a valley wind
in the creek or down the slope, thus acting as a wedge trigger to lift the warm dry air off the
ground.