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Understanding density altitude
The idea of density altitude begins with the standard atmosphere, a
table of air temperature, pressure and density at various altitudes. The actual
values of all of these change with the weather. But, the standard atmosphere
figures can be used to calculate for various altitudes how much lift a wing
should produce, how much power will come from the engine or engines and how much
thrust will push the aircraft along and how much drag should be produced.
Pilots need to adjust these theoretical values of lift, power and thrust to take
account of differences between the standard atmosphere and the real atmosphere
at a particular time and place. They use charts or aviation computers to say
that the real atmosphere at a particular time has the density of the standard
atmosphere at a certain altitude, which is likely to be different from the true
altitude. The aircraft performs as though it were at the density altitude.
To see how this works, look at our standard atmosphere table. Imagine that you
have some kind of device that directly measures the air's density. Imagine that
this device tells you the air's density is 0.001812 slugs per cubic foot. You'd
find that figure on the chart and then see that it's the density at 9,000 feet
in the standard atmosphere. We'd say that the aircraft was at a density altitude
of 9,000 feet, no matter what true altitude it's at.
In the U.S., such a density altitude is described as a "high density altitude."
This can be confusing because you might assume that the word "high" modifies
"density." That is, you might thank that the air has a high density as it does
at low altitudes. But, "high" really describes the altitude. In other words, the
air has a "high-altitude" density, which means the thin air decreases aircraft
performance.
Most density altitude charts and calculators account for the air pressure and
temperature, but not for humidity. Humid air is less dense than dry air, which
means performance will suffer on a humid day. But these effects are not as great
as temperature and air pressure. Our air density explained text has more on the
effects of humidity, along with the other factors that determine density.
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The following links are not part of USATODAY.com, but should help you understand
density altitude. If you are a pilot or student pilot, they should help you fly
safely by being aware of the effects on performance of high density altitude.
Use your browser's ''Back button'' to return to this page.
National Weahter Service, El Paso office: Density altitude calculator
Federal Aviation Administraion: Density altitude information
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The formula for density altitude
Tim Brice of the El Paso NWS office supplied us with the formula. He
also has the raw perl script files for the various programs on the El Paso
Weather Calculator, including the formula for density altitude, if you want to
program a computer to do the calculations. Click here to go to his index to
files with the scrips.
In the formula, / means to divide, * means to multiply, ** means the following
term is an exponent(i.e. X**0.235 means X to the 0.235 power), - means to
subtract, + means to add. The standard rules of algebra apply.
Density altitude = 145,366[1 - (X** 0.235 ) ]
where X = 17.326(Psta) / Temperature in Rankin degrees
Psta is the air pressure in inches of mercury. Use the actual peressure (the
station pressure) at the elevation (true altitude) that you are interested in.
The Temperature in Rankin is: Fahrenheit + 459.69
Brice notes that the temperature in the denominator should be virtual
temperature in degrees Rankin, which makes the calculation a little more
complicated than most want to deal with. Virtual temperature takes humidity into
account. If you do want to be this precise, and do the extra math, go to the
USATODAY.com Virtual temperature page for the formula.
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