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.
Air density is perhaps the single most important factor affecting aircraft performance. It has a direct bearing on:
- The lift generated by a wing — a reduction in the air density reduces the wing’s 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 an 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.
Aircraft taking off from a “hot and high” airport, such as the Quito Airport or Mexico City, are at a significant aerodynamic disadvantage. The following effects result from a density altitude that is higher than the actual physical altitude:
- An aircraft will accelerate more slowly on takeoff as a result of its reduced power production.
- An aircraft will need to achieve a higher true airspeed to attain the same amount of lift — this implies both a longer takeoff roll and a higher true airspeed, which must be maintained while airborne to avoid stalling.
- An aircraft will climb more slowly as a result of its reduced power production and reduced lift.
Due to these performance issues, an aircraft’s takeoff weight may need to be lowered, or takeoffs may need to be scheduled for cooler times of the day. The wind direction and the runway slope may need to be taken into account.
The density altitude is an important factor in skydiving, and one that can be difficult to judge properly, even for experienced skydivers. 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. Parachutes at higher altitudes fly more aggressively, making their effective area smaller, which is more demanding for a pilot’s skill and can be especially dangerous for high-performance landings, which require accurate estimates and have a low margin of error before they become dangerous.
The density altitude can be calculated from the atmospheric pressure and the outside air temperature (assuming dry air) using the following formula:
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The National Weather Service uses the following dry-air approximation to the formula for the density altitude above in its standard:
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This is an easier formula to calculate (with great approximation) the density altitude from the pressure altitude and the ISA temperature deviation:
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