I picked up a cheap infarred themometer a few days ago and discovered
that it will happily tell me the temperature of the clouds if they
are low.

It seems to me that the temp gradiant from the ground is well known
and if its consistant, this device combined with a table could tell
me how low the coulds are.

For example right now there are clear skys and its reading -27*C.
METAR at hte local airport is claiming 230900Z 19011KT CAVOK 14/09
Q1027 so there should be a 41*C difference between ground and
whatever the thing is reading the temp of in a clear sky. Today
it was reading -8 with a cloud base of at least 5000 ft.

Any comments on this?

-tim
http://web.abnormal.com

Tim Hogard opined

>I picked up a cheap infarred themometer a few days ago and discovered
>that it will happily tell me the temperature of the clouds if they
>are low.

>It seems to me that the temp gradiant from the ground is well known
>and if its consistant, this device combined with a table could tell
>me how low the coulds are.

>For example right now there are clear skys and its reading -27*C.
>METAR at hte local airport is claiming 230900Z 19011KT CAVOK 14/09
>Q1027 so there should be a 41*C difference between ground and
>whatever the thing is reading the temp of in a clear sky. Today
>it was reading -8 with a cloud base of at least 5000 ft.

>Any comments on this?

Clear sky should read 3*A, or about -270*C, the temperature of the cosmic
background radiation ;).

Cloud base temps should be surfaceTemp - altinThousands * 2. But I could be
wrong.


-ash
Cthulhu in 2005!
Why wait for nature?

"Ash Wyllie" > wrote in message
...
>
> Cloud base temps should be surfaceTemp - altinThousands * 2. But I could
be
> wrong.
>

It will be wrong when the lapse rate is different from standard, itself a
significant indicator that something is afoot. When I read a METAR I often
compare the observed ceiling to the surface temp to see if it's close to the
2C/1000 rule or not. If they're lower than they "should be" then you can bet
there's a good chance of convection, turbulence, and generally fast-changing
conditions. At least in New England this really only holds during the warmer
months though.

-cwk.

C Kingsbury opined

>"Ash Wyllie" > wrote in message
...
>>
>> Cloud base temps should be surfaceTemp - altinThousands * 2. But I could
>be
>> wrong.
>>

>It will be wrong when the lapse rate is different from standard, itself a
>significant indicator that something is afoot. When I read a METAR I often
>compare the observed ceiling to the surface temp to see if it's close to the
>2C/1000 rule or not. If they're lower than they "should be" then you can bet
>there's a good chance of convection, turbulence, and generally fast-changing
>conditions. At least in New England this really only holds during the warmer
>months though.

Good point.



-ash
Cthulhu in 2005!
Why wait for nature?

"C Kingsbury" > wrote in message
ink.net...
>
> "Ash Wyllie" > wrote in message
> ...
>>
>> Cloud base temps should be surfaceTemp - altinThousands * 2. But I could
> be
>> wrong.
>>
>
> It will be wrong when the lapse rate is different from standard, itself a
> significant indicator that something is afoot. When I read a METAR I often
> compare the observed ceiling to the surface temp to see if it's close to the
> 2C/1000 rule or not. If they're lower than they "should be" then you can bet
> there's a good chance of convection, turbulence, and generally fast-changing
> conditions. At least in New England this really only holds during the warmer
> months though.
>
> -cwk.

Would you elaborate on this please? Are you talking about comparing the temp/dewpoint spread to the ceiling at
2C/1000?

Cheers,
John Clonts
Temple, Texas
N7NZ

<snip>
>>It will be wrong when the lapse rate is different from standard, itself a
>>significant indicator that something is afoot. When I read a METAR I often
>>compare the observed ceiling to the surface temp to see if it's close to the
>>2C/1000 rule or not. If they're lower than they "should be" then you can bet
>>there's a good chance of convection, turbulence, and generally fast-changing
>>conditions. At least in New England this really only holds during the warmer
>>months though.
>>
>>-cwk.
>
>
> Would you elaborate on this please? Are you talking about comparing the temp/dewpoint spread to the ceiling at
> 2C/1000?
>
I think he's suggesting a comparison between the observed ceiling and the number
of feet you get by dividing the temp/dewpoint difference by 2 and multiplying by
1000. (AGL in both cases.)

If the actual and predicted ceilings are different then the lapse rate is not 2
-- not standard. If the bases are lower, then the lapse rate is deduced to be
larger than 2. And vice versa.

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.




(Tim Hogard) wrote in news:cnv2tr$2n3d$1
@knotty.abnormal.com:

> I picked up a cheap infarred themometer a few days ago and discovered
> that it will happily tell me the temperature of the clouds if they
> are low.
>
> It seems to me that the temp gradiant from the ground is well known
> and if its consistant, this device combined with a table could tell
> me how low the coulds are.
>
> For example right now there are clear skys and its reading -27*C.
> METAR at hte local airport is claiming 230900Z 19011KT CAVOK 14/09
> Q1027 so there should be a 41*C difference between ground and
> whatever the thing is reading the temp of in a clear sky. Today
> it was reading -8 with a cloud base of at least 5000 ft.
>
> Any comments on this?
>
> -tim
> http://web.abnormal.com

"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 et>...>
> 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 et>...>
>> 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 et>...>
>>
>>>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.

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. 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.



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.
>
>
>

On 25 Nov 2004 12:02:42 -0600, Andrew Sarangan
> wrote:

> The difference between the
>environmental lapse rate and the adiabatic lapse rate is what determines
>the stability


Like I said...

wrote:

> On Thu, 25 Nov 2004 09:14:32 -0500, Matt Whiting
> > wrote:
>
>
>>>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
>
>
>
> Not true as far as I know, but irrelevant nevertheless.

You need to do some research then and learn.


> Adiabatic lapse rates, by themselves, tell nothing about stability.
> It's the air mass that is either stable or unstable, and one needs to
> know what the actual lapse rate of the air mass is in order to
> predict stbility or instability.

The "actual" lapse rate is an adiabatic lapse rate. The lifted index is
determined by raising a parcel of air adiabatically.


Matt

"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
om...
> "Mike Rapoport" > wrote in message
> et>...>
>> 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.

How can 2C/1000' tell you anything about stability? If the air is saturated
then 2C/1000 is unstable, absoluteley totally unstable. 2C/1000 in
saturated air is likely to be the inside of a thunderstorm. If the air is
unsaturated and the lapse rate is 3C/1000, the air is stable, totally stable
and smooth. 2C/1000 tells you nothing about stability unless you know
whether the air is saturated or not. The only numbers that tell you about
stability without knowing the vapor content are lapse rates above 3C/1000
(termed absolutely unstable) or below 1C/1000 (absolutely stable). Any
lapse rate between 1C and 3C/1000 is termed "conditionally stable".

> 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'.

Mike
MU-2

"Andrew Sarangan" > wrote in message
1...
> 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.

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.
>>
>>
>>
>

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
1...
>>> 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
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.

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
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.
>
> 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