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Standard atmosphere

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) is 26.92 “Hg (29.92 – 3) and 9 °C (15 °C – 6 °C).