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.
ISA assumes that at Mean Sea Level, air is totally dry, the temperature is +15 degrees Celsius, Pressure is 1013.2 hecta pascals, the density is 1225 grams per cubic meter and acceleration due to gravity is 9.80665 Meters per second square.