How high is that cloud?

"Andrew Sarangan" wrote in message
1...
Probably the greatest use of your device is for figuring out the lapse
rate rather than cloud height. Get the cloud height and surface
temperature from METAR, use your device to get the cloud temperature and
calculate the lapse rate. A lapse rate higher than 2C/1000' will
indicate an unstable atmosphere and bumpy flight conditions. Unlike the
temperatures from the winds aloft forecast, yours will be actual
temperature, not forecasted, and will be more accurate. Also, winds
aloft data forecast is not accurate for calculating stability near the
surface, as the temperature is omitted for the first 3000'.
Extrapolating between surface and 6000' may not accurately reflect the
instability close to the ground.

2C per thousand has nothing to do with stability. In unsaturated air,
2C/1000 is stable. In saturated air 2C/1000 is unstable. 2C/1000 is the
standard for calibrating altimeters, it has nothing to do with the real
atmosphere or stability.

Mike
MU-2

On Wed, 24 Nov 2004 15:33:59 GMT, "Mike Rapoport"
wrote:

2C per thousand has nothing to do with stability. In unsaturated air,
2C/1000 is stable. In saturated air 2C/1000 is unstable. 2C/1000 is the
standard for calibrating altimeters, it has nothing to do with the real
atmosphere or stability.

Mike
MU-2




Exactly.

Stability is a function of the actual lapse rate and the dry (or
moist) adiabatic lapse rate.

The dry adiabatic lapse rate is 1C per 100 meters or 5 1/2deg F per
1000 feet.

If the actual lapse rate is more than this, the air is unstable.

In other words, a parcel of air will rise as long as the air around it
is cooler than the parcel. The parcel will be 5 1/2 degrees F cooler
after rising 1000 ft. If the surrounding air at 1000 feet is still
cooler than the cooled parcel, the parcel keeps on rising. It will
keep on rising (and cooling) until the parcel is the same
temperature as the surrounding air, which has its own (different)
lapse rate.

That's why its smoother above cumulus clouds. The clouds mark the top
of the column of rising air.

"Mike Rapoport" wrote in message link.net...
2C per thousand has nothing to do with stability. In unsaturated air,
2C/1000 is stable. In saturated air 2C/1000 is unstable. 2C/1000 is the
standard for calibrating altimeters, it has nothing to do with the real
atmosphere or stability.

Mike
MU-2

Adiabatic lapse rate is 1C/1"Hg for moist air (depending on moisture
content) and 3C/1"Hg for dry air. 2C/1"Hg is a representative average
for somewhat moist but unsaturated air. It is still a useful indicator
of stability. I would not discount is as a completely meaningless
number. It is a useful reference, just like 29.92" and 15C.

Please explain how 2C/1000 is used in altimeter calibration. I did not
know altimeters had any temperature corrections.

On 24 Nov 2004 21:38:35 -0800, (Andrew
Sarangan) wrote:

"Mike Rapoport" wrote in message link.net...
2C per thousand has nothing to do with stability. In unsaturated air,
2C/1000 is stable. In saturated air 2C/1000 is unstable. 2C/1000 is the
standard for calibrating altimeters, it has nothing to do with the real
atmosphere or stability.

Mike
MU-2

Adiabatic lapse rate is 1C/1"Hg for moist air (depending on moisture
content) and 3C/1"Hg for dry air. 2C/1"Hg is a representative average
for somewhat moist but unsaturated air. It is still a useful indicator
of stability. I would not discount is as a completely meaningless
number. It is a useful reference, just like 29.92" and 15C.



Adiabatic lapse rate never changes. It is not a useful indicator of
stability. It tells you nothing about stability until you know the
actual lapse rate of the air mass in question.



Please explain how 2C/1000 is used in altimeter calibration. I did not
know altimeters had any temperature corrections.

wrote:

On 24 Nov 2004 21:38:35 -0800, (Andrew
Sarangan) wrote:


"Mike Rapoport" wrote in message link.net...

2C per thousand has nothing to do with stability. In unsaturated air,
2C/1000 is stable. In saturated air 2C/1000 is unstable. 2C/1000 is the
standard for calibrating altimeters, it has nothing to do with the real
atmosphere or stability.

Mike
MU-2

Adiabatic lapse rate is 1C/1"Hg for moist air (depending on moisture
content) and 3C/1"Hg for dry air. 2C/1"Hg is a representative average
for somewhat moist but unsaturated air. It is still a useful indicator
of stability. I would not discount is as a completely meaningless
number. It is a useful reference, just like 29.92" and 15C.




Adiabatic lapse rate never changes. It is not a useful indicator of
stability. It tells you nothing about stability until you know the
actual lapse rate of the air mass in question.

It depends on which adiabatic lapse rate is being considered. The dry
adiabatic lapse rate is basically constant, but the moist/wet rates are not.

Matt

wrote in
:

On 24 Nov 2004 21:38:35 -0800, (Andrew
Sarangan) wrote:



Adiabatic lapse rate is 1C/1"Hg for moist air (depending on moisture
content) and 3C/1"Hg for dry air. 2C/1"Hg is a representative average
for somewhat moist but unsaturated air. It is still a useful indicator
of stability. I would not discount is as a completely meaningless
number. It is a useful reference, just like 29.92" and 15C.



Adiabatic lapse rate never changes. It is not a useful indicator of
stability. It tells you nothing about stability until you know the
actual lapse rate of the air mass in question.


You should read up on lapse rates. There is dry adiabatic lapse rate, moist
(saturated) adiabatic lapse rate, average adiabatic lapse rate and even
more obscure ones like superadiabatic lapse rate and autoconvective lapse
rate. It is definitely not a constant, but the commonly accepted average
lapse rate is 2C/1000ft.

"Andrew Sarangan" wrote in message
om...

Adiabatic lapse rate is 1C/1"Hg for moist air (depending on moisture
content) and 3C/1"Hg for dry air. 2C/1"Hg is a representative average
for somewhat moist but unsaturated air.

No, moist but unsaturated air has an adiabatic lapse rate of 3 degC (i.e.
the unsaturated adiabatic lapse rate). The difference between that and
saturated comes not from the amount of water vapor in the air as a mixing of
properties, but from the latent heat produced when the water vapor
condenses, which only happens when the air becomes saturated. It's not a
progression, but a sharp difference when the water vapor starts to condense.

Julian Scarfe

That's interesting, but I find it strange that moisture content does not
change the adiabatic lapse rate. Moist air has a higher heat capacity than
dry air, so I would expect the adiabatic lapse rate of moist air (but
unsaturated) to be lower than dry air.




"Julian Scarfe" wrote in
:

"Andrew Sarangan" wrote in message
om...

Adiabatic lapse rate is 1C/1"Hg for moist air (depending on moisture
content) and 3C/1"Hg for dry air. 2C/1"Hg is a representative average
for somewhat moist but unsaturated air.

No, moist but unsaturated air has an adiabatic lapse rate of 3 degC
(i.e. the unsaturated adiabatic lapse rate). The difference between
that and saturated comes not from the amount of water vapor in the air
as a mixing of properties, but from the latent heat produced when the
water vapor condenses, which only happens when the air becomes
saturated. It's not a progression, but a sharp difference when the
water vapor starts to condense.

Julian Scarfe

"Andrew Sarangan" wrote in message
1...

That's interesting, but I find it strange that moisture content does not
change the adiabatic lapse rate. Moist air has a higher heat capacity than
dry air, so I would expect the adiabatic lapse rate of moist air (but
unsaturated) to be lower than dry air.

True enough, but the difference in heat capacity is small. At 15 degC,
saturated air is still less than 2% water vapor, and the heat capacity of
water vapor is less than twice that of air. So you'd expect the variation
to be no more than a percent or two. That pales in comparison with the
difference made by condensation of that water vapor.

Julian Scarfe

On 26 Nov 2004 09:53:14 -0600, Andrew Sarangan
wrote:


That's interesting, but I find it strange that moisture content does not
change the adiabatic lapse rate. Moist air has a higher heat capacity than
dry air, so I would expect the adiabatic lapse rate of moist air (but
unsaturated) to be lower than dry air.


It is. But the additional heat comes from the process of
condensation, not expansion.

So the moist adiabatic lapse rate is a combination of two things,
cooling by expansion, and heating by condensation.





"Julian Scarfe" wrote in
:

"Andrew Sarangan" wrote in message
om...

Adiabatic lapse rate is 1C/1"Hg for moist air (depending on moisture
content) and 3C/1"Hg for dry air. 2C/1"Hg is a representative average
for somewhat moist but unsaturated air.

No, moist but unsaturated air has an adiabatic lapse rate of 3 degC
(i.e. the unsaturated adiabatic lapse rate). The difference between
that and saturated comes not from the amount of water vapor in the air
as a mixing of properties, but from the latent heat produced when the
water vapor condenses, which only happens when the air becomes
saturated. It's not a progression, but a sharp difference when the
water vapor starts to condense.

Julian Scarfe