# ISA Calculations MET

These calculations are of immense importance as they will be used in various ATPLATPL —Airline Transport Pilot Licence subjects.

Firstly, you need to know **how to calculate the temperature in ISAISAISA —International Standard Atmosphere —International Standard Atmosphere.** Start from the premise that in ISAISAISA —International Standard Atmosphere —International Standard Atmosphere the temperature is +15 degrees at mean sea level and that it falls by 2 degrees per 1000' of ascent. Therefore, at 1000' the temperature is 15 - (1x2) = 13, at 2000' is 15 - (2x2) = 11 and at 3000' is 15 - (3x2) = 09. A shorthand way of writing it is:

ISAISA —International Standard Atmosphere Temperature = 15 - (number of thousands of feet x 2).

Secondly, you need to know **how to calculate the ISAISAISA —International Standard Atmosphere —International Standard Atmosphere deviation.** This is a measure of **how many degrees warmer or cooler it is than ISAISA —International Standard Atmosphere**. So at the surface, it could be 10 or 20 degrees. Note that while there are different temperatures, that both of these are 5 degrees different from ISAISAISA —International Standard Atmosphere —International Standard Atmosphere. Consequently, **the sign (plus or minus) of the ISAISAISA —International Standard Atmosphere —International Standard Atmosphere deviation is of critical importance.** Now most people would regard a temperature of 20 degrees as nice warm temperature - a **positive** thing, you might say - and you can use this to state that **actual temperatures warmer than ISAISAISA —International Standard Atmosphere —International Standard Atmosphere give a positive ISAISAISA —International Standard Atmosphere —International Standard Atmosphere Deviation.** Similarly, a temperature of 10 degrees is relatively cold, and you might say this is a **negative thing - actual temperatures colder than ISAISAISA —International Standard Atmosphere —International Standard Atmosphere give a negative ISAISAISA —International Standard Atmosphere —International Standard Atmosphere Deviation.**

To calculate the ISAISAISA —International Standard Atmosphere —International Standard Atmosphere deviation, **find the number of degrees difference between the actual air temperature and ISAISAISA —International Standard Atmosphere —International Standard Atmosphere.** Then, **if actual is less than ISAISA —International Standard Atmosphere, deviation is negative.** If **actual is greater than ISAISA —International Standard Atmosphere, deviation is positive.**

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