Any sailplane pilots?

KK

If there is a uniform lift distribution, and the thermal large enough, there
would be little advantage to one direction of turn over another. However,
if there is a small, strong core then turning against the rotation would
reduce the turn radius and make it easier to stay in the core. At least
that's how it seems to work for me.

Bill Daniels

"Ken Kochanski" wrote in message
om...
Bill,

The Weatherwise article also supports the view that thermals (if you
consider them weaker cousins of dust devils) have equal probability of
left or right spin.

"To summarize, the Coriolis force has little bearing on the sense of
rotation in dust devils--about half of them spin one way and half the
other. By contrast, the large-scale rotation in the vicinity of
tornadoes and the storms that spawn them is usually cyclonic,
influenced by the Coriolis force."

But, how much advantage will you get choosing the correct thermalling
direction ? Let's assume the thermal is 500' in diameter with a
uniform lift of 5 knots and the rotation speed at the 250' radius is
10 MPH.

KK


"Bill Daniels" wrote in message
hlink.net...
"Ken Kochanski" wrote in message
om...
This article from Weatherwise looks at the mechanisms that cause spin
in storms, dust devils, etc. The thermals we fly in typically form in
the high following a frontal passage ... the flow in a high is
clockwise ... could it cause most thermals to have a clockwise
rotations ?

http://www.weatherwise.org/qr/qry.02coriolistorn.html


Alas, there have been studies that have found an almost even population
of
left and right hand rotation with, perhaps, a small edge to the left
hand
rotation in the northern hemisphere. Coriolis effects are more likely
to be
seen on large scales - much larger than dust devils.

On one occasion I observed a very large dust devil over a dry lake in
California. The central thermal was rotating counter-clockwise but
ringed
by a dozen or more dust devils rotating clockwise in the shear layer at
the
edge of the large one - somewhat like planet gears around a sun gear.
The
smaller dust devils were more obvious than the large central one so a
casual
ground observer would think that the majority of dust devils that day
were
clockwise. You had to be airborne to see the larger pattern. It pays
to be
careful with observations.

When you can determine the direction of rotation from airborne trash or
dust, it pays to turn against it.

Bill Daniels

Many years ago when I was being taught to thermal,
ALL of my instructors here in the UK taught me to sense
which wing pitched up and to turn in that direction
in order to find and centre the thermal (subject to
no other glider already being established in the thermal
of course).

I would imagine that any benefit achieved by turning
against any airmass rotation in the thermal would be
greatly outweighed by turning into the thermal towards
the core in the initial turn and then sticking with
that direction (subject to the first turn being in
the correct direction).

My instructors also used to watch for any bias students
might show in their turns in order to ensure that they
are comfortably able to turn in either direction (and
therefore centre thermals as fast as possible).

I would imagine that any pilot who has a 'preferred'
turning direction (either becasue they like turning
in that direction or because they believe they will
benefit from some rotational effect) is going to be
slower to cente a thermal compared to a pilot of similar
ability who has no such preference.

Happy New Year





At 15:24 01 January 2004, Bill Daniels wrote:
KK

If there is a uniform lift distribution, and the thermal
large enough, there
would be little advantage to one direction of turn
over another. However,
if there is a small, strong core then turning against
the rotation would
reduce the turn radius and make it easier to stay in
the core. At least
that's how it seems to work for me.

Bill Daniels

'Ken Kochanski' wrote in message
. com...
Bill,

The Weatherwise article also supports the view that
thermals (if you
consider them weaker cousins of dust devils) have
equal probability of
left or right spin.

'To summarize, the Coriolis force has little bearing
on the sense of
rotation in dust devils--about half of them spin one
way and half the
other. By contrast, the large-scale rotation in the
vicinity of
tornadoes and the storms that spawn them is usually
cyclonic,
influenced by the Coriolis force.'

But, how much advantage will you get choosing the
correct thermalling
direction ? Let's assume the thermal is 500' in diameter
with a
uniform lift of 5 knots and the rotation speed at
the 250' radius is
10 MPH.

KK


'Bill Daniels' wrote in message
news:...
'Ken Kochanski' wrote in message
om...
This article from Weatherwise looks at the mechanisms
that cause spin
in storms, dust devils, etc. The thermals we fly
in typically form in
the high following a frontal passage ... the flow
in a high is
clockwise ... could it cause most thermals to have
a clockwise
rotations ?

http://www.weatherwise.org/qr/qry.02coriolistorn.html


Alas, there have been studies that have found an
almost even population
of
left and right hand rotation with, perhaps, a small
edge to the left
hand
rotation in the northern hemisphere. Coriolis effects
are more likely
to be
seen on large scales - much larger than dust devils.

On one occasion I observed a very large dust devil
over a dry lake in
California. The central thermal was rotating counter-clockwise
but
ringed
by a dozen or more dust devils rotating clockwise
in the shear layer at
the
edge of the large one - somewhat like planet gears
around a sun gear.
The
smaller dust devils were more obvious than the large
central one so a
casual
ground observer would think that the majority of
dust devils that day
were
clockwise. You had to be airborne to see the larger
pattern. It pays
to be
careful with observations.

When you can determine the direction of rotation
from airborne trash or
dust, it pays to turn against it.

Bill Daniels

Anonymous Anonymous wrote:

I would imagine that any pilot who has a 'preferred'
turning direction (either becasue they like turning
in that direction or because they believe they will
benefit from some rotational effect) is going to be
slower to cente a thermal compared to a pilot of similar
ability who has no such preference.

Perhaps, but I go with my preference only when there is no wing lift or
other indication (about 50% of the time), the most common situation I
encounter; consequently, about 75% of my circling is to the right,
unless I make an effort to do otherwise, such as before a contest.
--
-----
change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

"Anonymous Anonymous" wrote in
message ...
Many years ago when I was being taught to thermal,
ALL of my instructors here in the UK taught me to sense
which wing pitched up and to turn in that direction
in order to find and centre the thermal (subject to
no other glider already being established in the thermal
of course).

I would imagine that any benefit achieved by turning
against any airmass rotation in the thermal would be
greatly outweighed by turning into the thermal towards
the core in the initial turn and then sticking with
that direction (subject to the first turn being in
the correct direction).

My instructors also used to watch for any bias students
might show in their turns in order to ensure that they
are comfortably able to turn in either direction (and
therefore centre thermals as fast as possible).

I would imagine that any pilot who has a 'preferred'
turning direction (either becasue they like turning
in that direction or because they believe they will
benefit from some rotational effect) is going to be
slower to cente a thermal compared to a pilot of similar
ability who has no such preference.

Although I was taught the same in the UK, there are other techniques. One
method suggests that you will center more quickly by turning away from the
lifted wing and find the core 270degs later and that by
turning toward the lifted wing you will fish around for the core longer.

Frank Whiteley

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

"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

Hi Mike,

The latent heat of evaporation comes to the account of the water in the
pool. So no problem here.
Looking for thermals in a marshland with quite some water in it is not such
a good idea on blue days. The layer of air close to the ground just does not
heat up enough to become unstable looks like. However with some "hot spots"
and unstable meteo conditions there need not be a problem in forming
thermals over marshland, but usually these areas should better be avoided is
also my experience.

Karel, NL

"Mike Lindsay" schreef in bericht
...
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

Hi Bill,

The observations you mention are very interesting and your remark that it is
better to look for moisture rather then for temperature in detecting
thermals is certainly true I think.

Using a forward looking IR sensor and a proper means of display in a glider
may be an idea to work on. I am not sure wings are long enough to detect a
usable spanwise buoyancy.
I remember reading about detecting spanwise temperature gradiënts by the
Germans. As a matter of fact I used very sensitive thermocouples on my
Pik-20D several years ago to detect a direction to turn into when hitting
unstable air. However this was quite unsuccesfull and supports your idea
that it is better to look for humidity rather then temperature to locate
upgoing drafts.

Karel, NL


"Bill Daniels" schreef in bericht
link.net...

"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

We have a large swamp we usually have to cross each way when xcountry
and I think of as a heat sink while it is at a lower temp than ambient and a
heat radiater (thermals) when day is cooling down in afternoon
gary
"K.P. Termaat" wrote in message
.. .
Hi Mike,

The latent heat of evaporation comes to the account of the water in the
pool. So no problem here.
Looking for thermals in a marshland with quite some water in it is not
such
a good idea on blue days. The layer of air close to the ground just does
not
heat up enough to become unstable looks like. However with some "hot
spots"
and unstable meteo conditions there need not be a problem in forming
thermals over marshland, but usually these areas should better be avoided
is
also my experience.

Karel, NL

"Mike Lindsay" schreef in bericht
...
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

I think your way of thinking is correct. I have an equivalent experience for
wooded areas. During the day they do not work well, but at dawn when the
environment is cooling down they give off their accumulated heat and produce
thermals.

Karel, NL


"goneill" schreef in bericht
...
We have a large swamp we usually have to cross each way when xcountry
and I think of as a heat sink while it is at a lower temp than ambient and
a
heat radiater (thermals) when day is cooling down in afternoon
gary
"K.P. Termaat" wrote in message
.. .
Hi Mike,

The latent heat of evaporation comes to the account of the water in the
pool. So no problem here.
Looking for thermals in a marshland with quite some water in it is not
such
a good idea on blue days. The layer of air close to the ground just does
not
heat up enough to become unstable looks like. However with some "hot
spots"
and unstable meteo conditions there need not be a problem in forming
thermals over marshland, but usually these areas should better be
avoided
is
also my experience.

Karel, NL

"Mike Lindsay" schreef in bericht
...
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