Standard Atmosphere

The International Standard Atmosphere (ISA) is a model defining average atmospheric conditions used as a baseline for aircraft performance calculations and altimeter calibration. At sea level, ISA specifies a temperature of 15 degrees Celsius (59 degrees Fahrenheit), a pressure of 29.92 inches of mercury (1013.25 hPa), and a standard lapse rate of approximately 2 degrees Celsius per 1,000 feet. Deviations from standard conditions directly affect density altitude and therefore aircraft performance; non-standard temperatures and pressures must be accounted for in all performance planning.