thumb|right|375px|Density Altitude Computation Chart

The density altitude is the altitude relative to standard atmospheric conditions at which the air density would be equal to the indicated air density at the place of observation. In other words, the density altitude is the air density given as a height above mean sea level. The density altitude can also be considered to be the pressure altitude adjusted for a non-standard temperature.

Both an increase in the temperature and a decrease in the atmospheric pressure, and, to a much lesser degree, an increase in the humidity, will cause an increase in the density altitude. In hot and humid conditions, the density altitude at a particular location may be significantly higher than the true altitude.

In aviation, the density altitude is used to assess an aircraft's aerodynamic performance under certain weather conditions. The lift generated by the aircraft's airfoils, and the relation between its indicated airspeed (IAS) and its true airspeed (TAS), are also subject to air-density changes. Furthermore, the power delivered by the aircraft's engine is affected by the density and composition of the atmosphere.

Aircraft safety

Air density is perhaps the single most important factor affecting aircraft performance. It has a direct bearing on:

  • The ability of a wing to create lift.
  • The efficiency of a propeller or rotor – which for a propeller (effectively an airfoil) behaves similarly to lift on a wing.
  • The power output of a normally-aspirated engine – the power output depends on the oxygen intake, so the engine output is reduced as the equivalent dry-air density decreases, and it produces even less power as moisture displaces oxygen in more humid conditions. In addition to the general change in wing efficiency that is common to all aviation, skydiving has additional considerations. There is an increased risk due to the high mobility of jumpers (who will often travel to a drop zone with a completely different density altitude than they are used to, without being made consciously aware of it by the routine of calibrating to QNH/QFE). Another factor is the higher susceptibility to hypoxia at high density altitudes, which, combined especially with the unexpected higher free-fall rate, can create dangerous situations and accidents.