Any sailplane pilots?

As a horticulturalist and a glider pilot, perhaps the answer is that the
moist ground acts as more of a heat sink into the ground, up until it
achieves a full heat load and then dissipates this in the afternoon, more so
than the surrounding dry ground. Vegetation on a west facing slope is a
great source in the afternoons out our way.

I have found lift in moister areas, but much more so in the afternoons,
mainly when lower in the convection zone and certainly not in the mornings.
Try walking around a wet area at 10am and 6pm and compare the relative heat
to dry areas.

The other issue that could be occurring is the bubble of cool air over the
moist ground could be acting as a trigger point, for the drifting heated air
from the surrounding dry areas. We all know of the tremendously small things
that can act as a trigger.

As to lapse rates the air is not saturated till cloud base, unless of course
a fog is present (mornings) which is a different case than trying to get a
thermal.

Robert P
Nimbus 2C

"Mike Rapoport" wrote in message
hlink.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

"goneill" wrote in message
...

"Aspley Nursery" wrote in message
...
Robert P
Nimbus 2C

I see in your sig Nimbus 2 C
That model is one I have been considering for some time.
Do you have any comments on it in comparison to other open class
gliders and in particular its approaches with the ventus style brakes.
Because there are none here I have some contrary comments from
other club members and even considered just getting a ASW20
instead .
The main reason I have been looking at it is most of the better pilots
are flying ventus's 18m but my budget only runs to an older open class
How does the 2c keep up with them all be it with a bit of water on to
get the wing loading up.The main "anti" comment is that it won't keep
up with its thicker aerofoil section and thus negating the main reason
why I am considering one.
gary

"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?

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?

If that were true, you would also find a VERY low cloud over the same spot.

Rgds,

Derrick.

Due to mixing with dryer air the humidity is spread out after a while and
condensation does not take place.

Karel, NL


"Derrick Steed" schreef in bericht
...
If that were true, you would also find a VERY low cloud over the same
spot.

Rgds,

Derrick.

"Andrew Sarangan" wrote in message
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?

No. The reason that saturated air lapses at a slower rate is that latent
energy is being released as the water vapor changes to liquid, that is the
only reason.

Mike
MU-2

I don't disagree with your math but the air over the water is also cooler
than the air over the dry land adjacent to the pond.

Mike
MU-2

"K.P. Termaat" wrote in message
...
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?

Reply to KP Termaat "Due to mixing with dryer air the humidity is spread out
after a while and
condensation does not take place".
I don't agree - common models of thermal convection are based on the
assumption that the air parcel (or column for that matter) rises
"adiabatically". If you refer to any text on Thermodynamics you will find
that "adiabatic" means that there is no exchange of energy with the
surroundings, "mixing with drier air" would invalidate that assumption.
My point was that any mixing of water vapour with the air above the pond
would certainly not take the humidity in the parcel as high as 60% except in
a region very close to the surface of the water - this may have something to
do with the "thermal" not sticking to the surface (is there such a thing as
the surface tension of a thermal bubble?) as much as it would over a dry
surface.
Rgds,
Derrick.

I've followed most of this thread and have found it interesting. I
think Mike Borgelt's analysis is a good accounting of what's going on.
I get to deal with underground mine ventilation and it has some
interesting analogies to this situation, water vapor and thermals. In
some situations moist air underground will be lighter than air in
other parts of a mine or tunnel and the difference in the "wieght"
will be sufficient to change the air flow - just because of humidity!
Most people, and pilots too - are surprised that dry air is heavier
than air with some moisture in it. A parcel at a given temperature
will be the lightest at 100% RH. I've been in thermals that I'm sure
originated in stock tanks or isolated ponds out west where the air is
"relatively" dry. It doesn't occurr in New England though, I'd guess
because there isn't enough of a humidity differential between the two
air parcels ( over the water, and adjacent ) and thier relative
humidities. Out west where the air is drier this does occurr and could
very well trigger a thermal. And then probably in conditions where
land forms or the area is conducive to preventing mixing with
surounding air by wind - a sheltered area or no air movement to allow
a parcel to build above a water source. And then the bouyant
difference in the air masses due to the humidity diferences has to be
enough to overcome the air cooling near the surface of the water ( the
swamp cooler thing! ) by the evaporation taking place into the air
above the water. Clear as mud - Mark Guay