U.S. Standard Atmosphere

It is largely consistent in methodology with the International Standard Atmosphere, differing mainly in the assumed temperature distribution at higher altitudes.

The USSA mathematical model divides the atmosphere into layers with an assumed linear distribution of absolute temperature T against geopotential altitude h.[2] The other two values (pressure P and density ρ) are computed by simultaneously solving the equations resulting from: at each geopotential altitude, where g is the standard acceleration of gravity, and Rspecific is the specific gas constant for dry air.

Air density must be calculated in order to solve for the pressure, and is used in calculating dynamic pressure for moving vehicles.

To allow modeling conditions below mean sea level, the troposphere is actually extended to −2,000 feet (−610 m), where the temperature is 66.1 °F (18.9 °C), pressure is 15.79 pounds per square inch (108,900 Pa), and density is 0.08106 pounds per cubic foot (1.2985 kg/m3).

The basic assumptions made for the 1962 version were:[3] The Federal Aviation Regulations define Standard Atmosphere in 14 CFR 1.1 by reference to the U.S. Standard Atmosphere, 1962 (Geopotential altitude tables).

Comparison of the 1962 US Standard Atmosphere graph of geometric altitude against air density , pressure , the speed of sound and temperature with approximate altitudes of various objects. [ 1 ]
Visualization of composition by volume of Earth's atmosphere. Water vapor is not included, as this is highly variable. Each tiny cube (such as the one representing krypton) has one millionth of the volume of the entire block. Data is from NASA Langley .