"K.P. Termaat" wrote

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


I don't know how it influences the analysis but, for Arizona, ambient
temp of 40 plus deg C and ambient humidity of about 15 percent are
more typical than the figures you used. Actual surface temperatures
probably run close to 60 C on a hot day.

I agree with others that the humidity concontinuity is probably the
trigger mechanism. Once the thermal has started it pulls in all the
surrounding super heated dry air.


Andy (GY)