Standard atmosphere

The atmosphere defined in U.S. Standard Atmosphere, 1962 (Geopotential altitude tables). (FAR definition).

At sea level, the standard atmosphere consists of a barometric pressure of 29.92 inches of mercury ("Hg) or 1013.2 millibars, and a temperature of 15 °C (59 °F). Pressure and temperature normally decrease as altitude increases. The standard lapse rate in the lower atmosphere for each 1,000 feet of altitude is approximately 1 "Hg and 2°C (3.5°F). For example, the standard pressure and temperature at 3,000 feet mean sea level (MSL) are 26.92"Hg (29.92"Hg – 3"Hg) and 9°C (15°C – 6°C). (PHAK definition).

Source: 14 CFR Federal Aviation Regulations §1, Pilot Handbook of Aeronatical Knowledge