Thread:Therefir/@comment-24503208-20180418194659/@comment-24503208-20180419005521

It was even explicitly noted here:

“The ISA is based 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.”