We have a large swamp we usually have to cross each way when xcountry
and I think of as a heat sink while it is at a lower temp than ambient and a
heat radiater (thermals) when day is cooling down in afternoon
gary
"K.P. Termaat" wrote in message
.. .
Hi Mike,

The latent heat of evaporation comes to the account of the water in the
pool. So no problem here.
Looking for thermals in a marshland with quite some water in it is not
such
a good idea on blue days. The layer of air close to the ground just does
not
heat up enough to become unstable looks like. However with some "hot
spots"
and unstable meteo conditions there need not be a problem in forming
thermals over marshland, but usually these areas should better be avoided
is
also my experience.

Karel, NL

"Mike Lindsay" schreef in bericht
...
In article , K.P. Termaat
writes

"Bob Salvo" schreef in bericht
...
Warm breeze picks up moisture at upwing edge of pond. Warm moist air
being
lighter than dry warm air, begins to rise, initiating thermal.

Happy New Year!
Bob

Yes, I agree Bob, Karel, NL

Mike Borgelt wrote:
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.

Just a simple approach with rough figures to support Mike's statement
and
hopefully to trigger the "smart guys".
At atmospheric pressure (say 1013 hPa) and at 20 C the density of dry
air
is about 1.22 kg/m3. Pure water vapor at atmospheric pressure has a
density
of 18/28 x 1.22 = 0.785 kg/m3, or 785 g/m3.
Air is saturated with water vapor when it contains 25 g/m3 at 20 C°.
Assume a relative humidity of say 30% on a dry day. Then one cubic
meter
of
air contains 0.3 x 25 = 7.5 g of water vapor and the air has then a
density
of 1.2159 kg/m3. Assume further that over a shallow pond the humidity
of
the
air increases to 60% due to a serious evaporation from the pond. Then
the
air directly over the pond will contain 0.6 x 25 = 15.0 g/m3
corresponding
to an air density of 1.2118 kg/m3.
So one cubic meter of air having 60% humidity is 1.2159 - 1.2118=
0.0041
kg
lighter then air with a humidity of 30%. This 4.1 g/m3 does not look
much,
but compare this figure with the decrease in density when air is heated
up.
The temperature coëfficiënt of air is 0.0044 kg/m3 per °C at 20 °C,
meaning
that when air is heated up by one degree its density decreases with 4.4
g/m3.
So one may conclude that changing the relative humidity of air from 30%
to
60% has the same effect on buoyancy as raising the temperature of air
by
1
°C.
So it may be worthwhile indeed to search for a thermal over a shallow
pond
in a dry area when low as I stated earlier.

Karel, NL

But wouldn't the latent heat of evaporation cool the air more that the
1deg C?. In which case a pond wouldn't work. But WTHDIK?

About 15 miles east of our site there is a low-lying marshland area
about 40 miles across which is all cut up with rivers and drainage
canals. I remember reading in an soaring text of the 1970s (I think it
was New Soaring Pilot) that it was a good idea to avoid this area
because all that water would stop convection.

So I asked on of the most experienced club members about it; he said
he'd not had any difficulty finding thermals there. He should know, he
had several UK records.






--
Mike Lindsay