high altitude temperature

[eji]

I was wondering if someone could clarify something for me? In the performance section of an aircraft QRH,there are temperatures for a given pressure altitude. For example, ISA -10, ISA+10. My question is this: What is ISA -10 at a pressure altitude of FL300 and what is ISA +10 at the same altitude? I'm getting mixed up with the double negative rule in addition and would like to clarify this once and for all. <img src="confused.gif" border="0">

[John Farley]

Hi

Not quite sure which bit is bothering you so excuse me if I state things you are happy with

ISA stands for International Standard Atmosphere

As such it is a purely arbitrary basis for setting out roughly what the real and variable atmos is like (as opposed to what the met man measures it as at any time).

Speaking from memory, at sea level the pressure in the ISA is defined as 1013.2 mb and the temperature as 15 deg C. If the met man measures the temp today at sea level and and it is 5 deg C then today is an ISA -10 day. If it was 25 deg C then it would be ISA + 10

So to use chart lines showing ISA + 10 and ISA - 10 amounts you first need to compare what the met man has measured/forecast the temp or where you are flying with the ISA value for the same height.

The lapse rate assumed in the ISA atmos is 1.98 deg C per 1000 ft up to 36000ft and zero after that (I think but I am too lazy to go and check).

But I am sure you have a table giving all the ISA values of pressure and temperature against height. If not that is where you must start ie get yourself one from just about any book dealing with flying.

Finally if you are happy that (say) your actual conditions at FL300 or wherever are ISA+7, then you look on the chart between ISA and ISA + 10 and estimate 70% of the way from the ISA line towards the ISA + 10 line and read off the value of whatever it is your chart is for.

[John Farley]

Just got off my but and opened a book

ISA temp at 30000ft is -44.4 deg C so ISA + 10 at that height is 10 deg warmer = 34.4 deg C and ISA - 10 is 10 deg colder so -54.4 deg C

[eji]

Thanks John, I suspected that to be true but wasn't 100% sure. Once again thanks for the input. <img src="smile.gif" border="0">

[john_tullamarine]

A useless bit of extra information .. I recall reading a long time ago that the ISA profiles were based on research met observations conducted in the US continental mid latitudes and, as such, purport to represent temperate latitude, non-extreme averages ....

[GlueBall]

...And be mindful of the fact that Minus 40 Fahrenheit equals Minus 40 Centigrade! <img src="tongue.gif" border="0">

[Ready]

ISA Temp is +15 Centigrade,. .ISA lapse rate is 2 degrees Centigrade (1.98 for the purists) per thousand feet.

So take your altitude times 2 (for 2 degrees C.), minus the +15 degrees, which is the standard day at sea level.

Using somebody else's example for 30,000 feet: 30X2=60-15=45 degrees. Numbers of degree(s) on either side is your deviation from ISA. I think the exact number was 44.6 degrees, probably using 1.98 instead of 2 degrees. I can live with that error, can you? Quick and easy.

Enjoy.

[Ready]

Forgot to mention something very important:

Standard tropopause temperature is -56.5 at 36,200 feet. This is a standard atmosphere and it does not mean that if your OAT shows -56.5 degrees, that you are at the tropopause. At southerns latitudes where the the Tropopause is much higher you will find much colder temperatures than that for the trop. Near the equator, the tropopause is at its coldest. Let's say today the tropopause where your airplane actually flies is 45,000 feet, trop. temperature could be close to -75 degrees (45x2-15). You see, we're very far from our -56 degrees, which is only a standard, just like 15 and 1.98 degrees are.

So using your QRH, when above 36,000, whether you're at 37,000 or 45,000 feet, with an OAT of -66 degrees, you're ISA-10, because it was established long time ago that in a standard atmosphere, the temperature does not change at all above 36,200 feet. So I use my formula (altx2-15) up to 36,000 feet, above I use -55 degrees for quick reference.

Cheers!!!