Standard Atmosphere
What is standard atmosphere?
The standard atmosphere is a hypothetical model of the atmosphere used as a reference for calibrating instruments and comparing aircraft performance. At sea level it assumes a pressure of 29.92 inches of mercury (inHg), a temperature of 15 °C (59 °F), and a standard lapse rate as you climb (temperature decreasing and pressure dropping with altitude). Think of it as the "textbook" atmosphere—real conditions rarely match it exactly, but it gives everyone a common baseline.
Also known as: International Standard Atmosphere, ISA, standard day, U.S. Standard Atmosphere
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