Density-Altitude

By Paul Quade

As spring rapidly turns to summer it's probably a good time to review the concept of density-altitude.

Density-altitude is a term to describe how dense the atmosphere is in comparison to the International Standard Atmosphere. The datum point for ISA is 59 degrees Fahrenheit (15C), with a pressure of 29.92 inches of mercury (1013mb) at sea level. The following chart shows the ISA temperature values for some different altitudes above mean sea level.



      MSL        F        C 
     8000     31F     -1C 
     7000     34F      1C 
     6000     38F      3C 
     5000     41F      5C 
     4000     45F      7C 
     3000     48F      9C 
     2000     52F     11C 
     1000     55F     13C 
Sea Level     59F     15C 


Air density decreases as altitude increases. A rough rule of thumb is that with about every 18,000 to 20,000 ft of altitude gained, there is about half as much air density. So at about 40,000 ft, there is about one quarter the amount of molecules per cubic centimeter of air as found at sea level.

Air density varies with temperature. For every 15F or 8.5C variation from the standard temperature at your pressure altitude, the density-altitude is increased or decreased by 1000 ft.

To create the same amount of lift, for each 1000 ft increase in density-altitude the true airspeed will increase by 2 percent.

Since kinetic energy increases at the square of the velocity, for each 1000 ft increase in density-altitude, the landing rollout (or the energy required to be absorbed by your legs) will increase by 4 percent.

Going from a low altitude DZ like Monterey, California in the winter to a high DZ like Mile-Hi Colorado in summer, be aware that your canopy will land quite differently (a lot faster)!

These effects can even be seen at a lower altitude DZ like Perris, California when it starts to get, uh, "warm". Landing at Perris (1410 MSL) in the summer with a temperature of 105F can be the equivalent of landing at a DZ located over 5,000 feet above sea level on a "normal" day.

Humidity is not much of a factor in determining the landing effects of density-altitude. Humidity is a factor for pilots during takeoff as it affects engine performance.


Paul Quade - Do not reproduce without permission.
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