"Andy Durbin" schreef in bericht
om...
"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)

The additional buoyance of the air over the pond is caused by the change in
humidity of this air. In my example I used a change of 30% in humidity
causing an equal effect as heating up the air by an additional one degree C.
With very low humidity to start with, e.g. the 15% you mention, it may be
possible that the change in humidity is more then 30% causing a somewhat
larger decrease in air density. The higher ambient temperatures have also a
positive effect on this, so without going through the calculations once
more it looks like the water vapor effect is stronger for your Arizona case.

Karel, NL