Any sailplane pilots?

If that were true, you would also find a VERY low cloud over the same spot.

Rgds,

Derrick.

Due to mixing with dryer air the humidity is spread out after a while and
condensation does not take place.

Karel, NL


"Derrick Steed" schreef in bericht
...
If that were true, you would also find a VERY low cloud over the same
spot.

Rgds,

Derrick.

Reply to KP Termaat "Due to mixing with dryer air the humidity is spread out
after a while and
condensation does not take place".
I don't agree - common models of thermal convection are based on the
assumption that the air parcel (or column for that matter) rises
"adiabatically". If you refer to any text on Thermodynamics you will find
that "adiabatic" means that there is no exchange of energy with the
surroundings, "mixing with drier air" would invalidate that assumption.
My point was that any mixing of water vapour with the air above the pond
would certainly not take the humidity in the parcel as high as 60% except in
a region very close to the surface of the water - this may have something to
do with the "thermal" not sticking to the surface (is there such a thing as
the surface tension of a thermal bubble?) as much as it would over a dry
surface.
Rgds,
Derrick.

I've followed most of this thread and have found it interesting. I
think Mike Borgelt's analysis is a good accounting of what's going on.
I get to deal with underground mine ventilation and it has some
interesting analogies to this situation, water vapor and thermals. In
some situations moist air underground will be lighter than air in
other parts of a mine or tunnel and the difference in the "wieght"
will be sufficient to change the air flow - just because of humidity!
Most people, and pilots too - are surprised that dry air is heavier
than air with some moisture in it. A parcel at a given temperature
will be the lightest at 100% RH. I've been in thermals that I'm sure
originated in stock tanks or isolated ponds out west where the air is
"relatively" dry. It doesn't occurr in New England though, I'd guess
because there isn't enough of a humidity differential between the two
air parcels ( over the water, and adjacent ) and thier relative
humidities. Out west where the air is drier this does occurr and could
very well trigger a thermal. And then probably in conditions where
land forms or the area is conducive to preventing mixing with
surounding air by wind - a sheltered area or no air movement to allow
a parcel to build above a water source. And then the bouyant
difference in the air masses due to the humidity diferences has to be
enough to overcome the air cooling near the surface of the water ( the
swamp cooler thing! ) by the evaporation taking place into the air
above the water. Clear as mud - Mark Guay

From my experience it appears possible for thermals
to form over and rise from a body of water. In the
past I have done a great deal of hang glider ridge
soaring on the Eastern edge of Lake Michigan. The ridge
is a steep sand bluff about 300-400 feet high right
on the edge of the water. The water is so close that
at some points it touches the base of the bluff. The
lake at this point is about 80 miles wide. On rare
occasions during light winds just capable of sustaining
flight we would encounter what were believed to be
thermals. The water would be almost perfectly flat
and you could see circular disturbances on the surface
moving towards shore at wind speed. When they hit the
bluff you could circle up and drift back over the ridge.
Our only explanation was clouds that caused uneven
heating on the surface of the water were creating the
thermals.