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