# 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 AtmosphereInternational Standard Atmosphere. Start from the premise that in ISAISAISA —International Standard AtmosphereInternational 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 AtmosphereInternational 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 AtmosphereInternational Standard Atmosphere. Consequently, the sign (plus or minus) of the ISAISAISA —International Standard AtmosphereInternational 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 AtmosphereInternational Standard Atmosphere give a positive ISAISAISA —International Standard AtmosphereInternational 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 AtmosphereInternational Standard Atmosphere give a negative ISAISAISA —International Standard AtmosphereInternational Standard Atmosphere Deviation.

To calculate the ISAISAISA —International Standard AtmosphereInternational Standard Atmosphere deviation, find the number of degrees difference between the actual air temperature and ISAISAISA —International Standard AtmosphereInternational 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.