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Atmosphere

12 Oct

Atmosphere

Air density is a result of the relationship between temperature  and pressure. This relationship is such that density is  inversely related to temperature and directly related to pressure. For a constant pressure to be maintained as temperature increases, density must decrease, and vice versa. For a constant temperature to be maintained as pressure increases, density must increase, and vice versa.

 

Standard Atmosphere

In the standard atmosphere, sea level pressure is 29.92″ Hg and the temperature is 15 °C (59 °F). The standard lapse rate for pressure is approximately a 1″ Hg decrease per 1,000 feet increase in altitude. The standard lapse rate for temperature is a 2 °C (3.6 °F) decrease per 1,000 feet increase, up to the tropopause.

 

Pressure Altitude

There are two measurements of the atmosphere that pilots must understand: pressure altitude and density altitude.

 

Density Altitude

Density altitude is pressure altitude corrected for nonstandard temperatures, and is used for determining aerodynamic performance in the nonstandard atmosphere.   However, a known density occurs for any one temperature and pressure altitude combination.

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