Definition

Density altitude is pressure altitude corrected for temperature. In layman's terms it directly affects the performance parameters of any aircraft, and in effect it is the equivalent altitude of where, performance-wise, the aircraft “thinks” it's at. The higher the density altitude, the lower the aircraft performance, and vice versa.

Density and pressure/temperature

The Ideal Gas Law for dry air:

pV = RT

where:

p = air pressure (hPa) (kg m^-1 s^-2) V = air volume (m³) R = universal gas constant for dry air (287 J kg^-1 K^-1) T = air temperature (K)

For atmospheric applications, however, volume is not well defined, and since density is mass per volume, using atmospheric mass and volume, the Gas Law can be reformulated:

p = ρRT where ρ = air density

Therefore

ρ = p R^-1 T^-1

Density is directly proportional to pressure and indirectly proportional to temperature.