International Standard Atmosphere

In this post, we shall understand about the need and details of International Standard Atmosphere which is also referred as ISA Conditions or ISO Conditions. Earlier we had understood about all the layers in the atmosphere including the Ionosphere which has a large amount of free Ions.


Most of our flying and our weather is contained within the first two layers of the atmosphere, namely the troposphere and the stratosphere. Within these two layers we have many properties which are important to us like density, pressure and temperature.

It is essential to construct a model atmosphere for calibration of instruments and aircraft testing. This model atmosphere is known as the International Standard Atmosphere shortened as ISO or ISA. This model extends to 32 kilometres above the surface of earth.


ISA assumes that at Mean Sea Level, Air is Totally Dry, the Temperature is +15 Degrees Celsius, Atmospheric Pressure is 1013.2 Hecta Pascals, the Density of Air is 1225 Grams per Cubic Meter and the Acceleration due to Gravity is 9.80665 Meters per Second Square.

These ISA conditions are used for Calibrations of all the Flight Instruments like Altimeter, Air Speed Indicator, Machmeter and many more such instruments. Having learnt about ISA Conditions we shall proceed to learn about ISA lapse Rate and its effects in Aviation Meteorology.