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Temperature Deviation

Sat Mar 30, 2002 12:15 pm

What purpose does temperature deviation information serve to pilots?

My understanding is the temperature deviation is the difference between the temperature at altitude and ISA. And my understanding of ISA is 15 degrees celcius at sea level with each 1000 feet decreasing in two degrees of temperature. Can someone please confirm or deny this formula?

Thank you,

RE: Temperature Deviation

Sat Mar 30, 2002 4:15 pm

I can confirm it. It's what's called the International Standard Atmospheres lapse rate (ISA LR). And you are right, it is 2 degrees C per 1000 feet. This is very useful in estimating freezing levels.

What the lapse rate (LR) is is actually a measure of atmospheric stability.

The actual lapse rate varies from airmass to airmass, and is determined by this formula:

LR=Temp(bottom)-Temp(top)/Thickness of layer

When you compare it to the Dry Adiabatic Lapse Rate (DALR), you can tell how stable or unstable the airmass is. The DALR is a constant 3 degrees C per 1000 ft.

LR > DALR = Unstable atmosphere
LR = DALR = Nuetral atmosphere
LR < DALR = Stable

We can get into why this is if you want, but it's sort of outside the scope of the original question.

Remember one thing though: Temperature and Dewpoint don't converge at the ISA LR. They have their own. It's 2.4 degrees C per 1000 feet. This is very useful in estimating cloud bases.

And regarding the term "temperature deviation," I've never heard it before, and I can't find it in the glossaries of my weather books. But, in answer to your question, it's really quite important to know about the temperature. It goes into computing, well a lot of critical information, such as takeoff/landing distance, density altitude, true airspeed, and the list goes on.

Hope this helps. Fire away if you need me to clear anything up.

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Joined: Sat Feb 16, 2002 11:46 am

RE: Temperature Deviation

Sun Mar 31, 2002 1:17 am

Some aircraft performance charts use temperature deviation (diference + or - from standard at that altitude) to adjust for the affect of temperature. The numbers will be for standard day and then there will be a conversion factor for that altitude for every degree above or below standard.

RE: Temperature Deviation

Sun Mar 31, 2002 1:24 am

Since the air mass is denser at lower temperatures, the temp deviation is important in all performance calculations. We use the temp dev. to determine cruise speeds, cruise altitudes and fuel consumption. As an example, and aircraft may be able to climb to FL 330 on a standard day, with a positive temp dev. the same aircraft may only be able to climb to FL 310. IF you look at a cruise performance chart you will see a correction for temp dev. Usually you have an ISA line, then you will see ISA +5, ISA +10 etc.. Most will also give ISA -5, ISA -10 and so forth. A lot of pilots won't include a negative temp deviation, it's considered a pad.
Hope this helps.
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Joined: Fri Sep 17, 1999 7:43 am

Actual ISA

Tue Apr 02, 2002 3:16 am

The ISA is actually based on meters, of course. ISA temp at sea level is 15 deg C and drops at a constant rate to 11000 meters where ISA temp is minus 56.5. So lapse rate is exactly 6.5 degrees C per thousand meters. Dunno whether that's geometric or geopotential height, tho.

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