As defined by ISA, the Mean Sea Level temperature is +15 degrees centigrade. The temperature decreases at a constant rate of 1.98 degrees Celsius per 1000 feet, or by 0.65 degrees Celsius per 100 meters. The temperature continues to reduce down to minus 56.5 degrees Celsius at 11 kilometres above the earth’s surface.

This is where we find the tropopause in ISA, where the temperature no longer decreases with altitude. From this point onwards we have the tropopause. In tropopause, ISA assumes the temperature would remain constant at minus 56.5 degrees Celsius up to 20 kilometres or 65,617 feet.

From 20 kilometres to 32 kilometres above the surface we see a gradual increase in temperature by 0.3 degrees Celsius per 1000 feet, or 0.1 degree Celsius per 100 meters. If you remember, this is the part of the stratosphere that contains the gas ozone, which causes the surrounding air to warm.

All aircraft instruments are calibrated to ISA. You need to understand that all aircraft instruments would indicate correctly only under ISA conditions. Therefore, it’s essential to know just how much the actual atmosphere on a given day differs from ISA. This would enable us to apply the required corrections.

The simplest method we use to analyse this difference is by comparing the prevailing temperature to the standard temperature as per ISA. This is known as the ISA deviation. In other words, how much the real atmosphere deviates from the standard atmosphere.

Remember, we had said that ISA assumes that the sea level temperature is +15 degrees Celsius. The temperature then fell by 1.98 degrees Celsius per 1000 feet. At an airfield 1000 feet above sea level, the ISA temperature should be 13.02 degrees Celsius.

Let us say that the actual temperature is 20 degrees Celsius, which is warmer than ISA. A simple mathematical calculation shows that the atmosphere is 6.98 degrees Celsius warmer than ISA.

We can say that the real atmosphere is +6.98 which means that the actual temperature is 6.98 degrees Celsius warmer than that of ISA. This is called as the ISA Deviation. The formula for ISA deviation is ISA DEVIATION = ACTUAL TEMPERATURE – ISA TEMPERATURE