How high is that cloud?

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

"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

Mike Rapoport wrote:

"Andrew Sarangan" wrote in message
om...

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


Altimeters are preasure guages with a scale in feet. They assume a pressure
lapse rate with altitude and the pressure lapse rate that they use is ISA
whch assumes 15C at SL and 2C/1000'.

Last I knew C was a unit of temperature, not pressure.

Matt

"Matt Whiting" wrote in message
...
Mike Rapoport wrote:

"Andrew Sarangan" wrote in message
om...

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


Altimeters are preasure guages with a scale in feet. They assume a
pressure lapse rate with altitude and the pressure lapse rate that they
use is ISA whch assumes 15C at SL and 2C/1000'.

Last I knew C was a unit of temperature, not pressure.

Matt


Last I heard temperature had an effect on the density of any gas with the
consequent lowering (in feet) of any preasure level with decreasing
temperature. Remember "high to low, lookout below". Canada even has
altitude correction tables for use when temps are very low.

What is the pressure at 1000' MSL at -50C? How about at +50C?

Mike
MU-2

wrote in message
...
On Fri, 26 Nov 2004 17:53:01 GMT, "Mike Rapoport"
wrote:


"Andrew Sarangan" wrote in message
.61...
Ah, I see what you are getting at. But I never said stability is only
determined by adiabatic lapse rate. The difference between the
environmental lapse rate and the adiabatic lapse rate is what determines
the stability. The average adiabatic lapse rate is 2C/1000' but can vary
based on moisture. Therefore, if the environmental lapse rate is higher
than 2C/1000' that atmosphere can be considered to be unstable.

Absolutely not! If the air is unsaturated and the lapse rate is 2C/1000
but
below 3C/1000, then the air is absolutely, totally stable! No "ifs",
"ands", or "buts", it is stable. It cannot be considered to be unstable.


If the air is unsturated, it will cool at the dry adiabatic lapse
rate. If it becomes saturatede, it will cool at the moist adiabatic
lapse rate. This alone has nothing to do with stabiity.

If the environmental lapse rate is greater (superadiabatic), the
atmosphere will be unstable (the parcel will rise). If the
environmental lapse rate is less than the adiabatic lapse rate, the
air will be stable. The parcel will not rise (an it might even sink).


Isn't that what I just said?

Mike
MU-2


For a
moist parcel of air 2C/1000 environmental lapse rate will be unstable,
but for a dry parcel of air it will be stable.

Yes! but this is in direct contradiction to your above statement
"Therefore, if the environmental lapse rate is higher
than 2C/1000' that atmosphere can be considered to be unstable."

Mike
MU-2

wrote in
:



Adiabatic lapse rates (there are more than one) are theoretical and
calculated.

The calculations do not change from day to day.

The dry adiabatic lapse rate witll be calculated tomorrow the same way
it is today. It measures a physical process, i.e., the amount of heat
given up as air rises and expands,

Therefore it does not, as you suggest, indicate stability or
instability, since it will be the same in unstable air as it is in
stable air.

It is the ;lapse rate of the surrounding air that determines
stability, not the lapse rate of the rising air, which is the same
every day, day in and day out.

Mike Rapoport wrote:

"Matt Whiting" wrote in message
...

Mike Rapoport wrote:


"Andrew Sarangan" wrote in message
e.com...

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


Altimeters are preasure guages with a scale in feet. They assume a
pressure lapse rate with altitude and the pressure lapse rate that they
use is ISA whch assumes 15C at SL and 2C/1000'.

Last I knew C was a unit of temperature, not pressure.

Matt



Last I heard temperature had an effect on the density of any gas with the
consequent lowering (in feet) of any preasure level with decreasing
temperature. Remember "high to low, lookout below". Canada even has
altitude correction tables for use when temps are very low.

It does, I'm not disputing that. However, a gas can have the same
pressure at multiple temperatures, so knowing the temperature alone
isn't sufficient to make a pressure calibration.


Matt

"Matt Whiting" wrote in message
...
Mike Rapoport wrote:

"Matt Whiting" wrote in message
...

Mike Rapoport wrote:


"Andrew Sarangan" wrote in message
le.com...

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


Altimeters are preasure guages with a scale in feet. They assume a
pressure lapse rate with altitude and the pressure lapse rate that they
use is ISA whch assumes 15C at SL and 2C/1000'.

Last I knew C was a unit of temperature, not pressure.

Matt



Last I heard temperature had an effect on the density of any gas with the
consequent lowering (in feet) of any preasure level with decreasing
temperature. Remember "high to low, lookout below". Canada even has
altitude correction tables for use when temps are very low.

It does, I'm not disputing that. However, a gas can have the same
pressure at multiple temperatures, so knowing the temperature alone isn't
sufficient to make a pressure calibration.


Matt

The altimeter measures only pressure. Since pressure at any given altitude
varies with temperature so there had to be a convention on what the standard
temperature lapse rate would be and that was ISA which assumes 2C/1000'. My
only point is that knowing that the actual atmospheric lapse rate is 2C/1000
tells nothing about stability since the air can be either stable or unstable
at that lapse rate.

Mike
MU-2