Any sailplane pilots?

Andrew, read the numerical support of Mike Borgelt's statement below which I
posted a few days ago.


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


"Andrew Sarangan" schreef in bericht
om...
"Mike Rapoport" wrote in message
thlink.net...
"Andrew Sarangan" wrote in message
om...
(Kirk Stant) wrote in message
. com...
"K.P. Termaat" wrote in message
...
My experience is that it works, especially on days with very low
humidity,
but no boomers and only low.


"Mike Rapoport" schreef in bericht
ink.net...

You will find less lift over water of any kind, even if it is
contained in
vegetation. The best lift is always over the highest, dryest,
darkest
surface around. The water vapor idea is...well...it is hard to
find
a
place
to start...but it won't work

Mike
MU-2


Have to disagree with you, Mike - out here in Arizona, in the desert
areas that are not irrigated, we often find good lift directly over
small cattle "tanks" - small shallow ponds that are scattered
around.
A lot of us have noticed this and compared notes, and it works; if
too
low to get to high, dark ground, I'll head for the nearest pond and
it
will usually turn up a nice thermal. We think it may be due to the
fact that the ponds are in a natural low spot, and coupled with the
little bit of moisture, could be the necessary trigger for a
thermal.

Now obviously, large irrigated farm fields or river basins are death
to thermals - but a local lake (reservoir) seems to have little
effect
on thermal activity - could it be all the drunk boaters?

What's the old saying about never saying never?

Kirk
LS6-b


The adiabatic rate of moist air is about 1.5C/1000ft. For dry air it
is 3C/1000ft. Therefore, assuming that the water temperature is the
same as the surrounding ground (which would be true if the water is
shallow), I could see how one will experience greater lift above
water.

Not for unsaturated air it isn't. The moist rate only applies to
saturated
air (ie in clouds)

Mike
MU-2


OK, 1.5C/1000ft applies only to 100% RH air. But a 50% RH air must
still have a lower lapse rate than dry air, no?