In our earlier lesson, we had seen that as per the definition of International Standard Atmosphere, the Mean Sea Level temperature is +15 degrees centigrade. In this post, let us understand the lapse rate of temperature as per ISA Conditions.

### ISA TROPOSPHERE – TROPOPAUSE

As per ISA conditions, we have defined that 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.

### ISA STRATOSPHERE

Above the height of 20 kilometres upto 32 kilometres above the earth’s 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.

### ISA DEVIATION

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 International Standard Atmosphere.

### EXAMPLE OF ISA DEVIATION

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.

### ISA DEVIATION FORMULA

ISA deviation can be expressed as a formula.

ISA DEVIATION = ACTUAL TEMPERATURE – ISA TEMPERATURE

In this post, we have understood about the calculation and importance of ISA Deviation. In our next post we shall understand a little about Jet Standard Atmosphere.