"Mike Lindsay" wrote in message
...
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

This supports something I saw back in the 1960's from instrumented airplane
traverses made at mid levels in strongly convective conditions . The data
clearly showed the updrafts corresponding to thermals but did not show any
temperature rise in the thermals. Instead, they showed an increase in
absolute humidity corresponding to an increase of about 30% in relative
humidity over the surrounding air.

It's easy to see that the relative humidity in a thermal steadily increases
with height above the ground until it reaches 100% at cloud base. The
source of most of this moisture has to be the earth's surface below so the
thermal is a transport mechanism that lifts water vapor up to cloud base.

This has always led me to think that people looking to invent remote thermal
sensors should not be looking for water vapor and not warm air. Water vapor
has interesting infrared absorption spectra that might allow IR Imaging of
thermals (it sure works well in weather satellite images). Wingtip mounted
wet bulb sensors would directly read the temperature + humidity which should
correspond nicely to the spanwise buoyancy gradient and should be a reliable
indicator of the best direction of turn when entering a thermal.

Bill Daniels