Also known as the ICAO Standard Atmosphere, ISA is a standard against which to compare the actual atmosphere at any point and time.
The ISA is based on the following values of pressure, density, and temperature at mean sea level each of which decreases with increase in height:
Pressure of 1013.2 millibar - Pressure is taken to fall at about 1 millibar per 30 feet in the lower atmosphere (up to about 5,000 feet).
Temperature of +15 °C - Temperature falls at a rate of 2 °C per 1,000 feet until the tropopause is reached at 36,000 feet above which the temperature is assumed to be constant at -57 °C. (The precise numbers are 1.98 °C, -56.5 °C and 36,090 feet)
Density of 1,225 gm/m3.
The real atmosphere differs from ISA in many ways. Sea level pressure varies from day to day, and there are wide extremes of temperature at all levels.
Variation in pressure, vertically and horizontally, affects the operation of the pressure altimeter.