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Density altitude is the altitude relative to the standard atmosphere conditions (ISA) at which the air density would be equal to the indicated air density at the place of observation. In other words, density altitude is air density given as a height above mean sea level. "Density altitude" can also be considered to be the pressure altitude adjusted for non-standard temperature.
Both an increase in temperature, decrease in atmospheric pressure, and, to a much lesser degree, increase in humidity will cause an increase in 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 the aircraft's aerodynamic performance under certain weather conditions. The lift generated by the aircraft's airfoils and the relation between indicated and true airspeed are also subject to air density changes. Furthermore, the power delivered by the aircraft's engine is affected by the air density and air composition.
Air density is perhaps the single most important factor affecting aircraft performance. It has a direct bearing on:
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 which is higher than the actual physical altitude:
Due to these performance issues, a plane's takeoff weight may need to be lowered or takeoffs may need to be scheduled for cooler times of the day. Wind direction and runway slope may need to be taken into account.
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 common for all aviation, skydiving has additional considerations. There is an increased risk due to the high mobility of jumpers (who will often travel to a dropzone 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, especially combined with the unexpected higher free fall rate, can create dangerous situations and accidents. Parachutes at higher altitudes fly more aggressively, making their effective area lower, which is more demanding for the 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.
Density altitude can be calculated from atmospheric pressure and temperature (assuming dry air).
National Weather Service Equation
The National Weather Service uses the following dry-air approximation of the above equation in their standards.
Note that the NWS standard specifies that the density altitude should be rounded to the nearest 100 feet.
Easy formula to calculate density altitude from pressure altitude
This is an easier formula to calculate (with great approximation) density altitude from pressure altitude ..and International Standard Atmosphere temperature deviation
considering that temperature drops at the rate of 1.98 °C each 1000 ft of altitude until the Tropopause (36000ft), usually rounded to 2ºC
Or even simpler
DA=1.24 PA + 118.8 OAT - 1782
where DA=density altitude and PA=pressure altitude where PA=Hgt+30(1013-QNH) and QNH = QNauticalHeight = Height above sea level