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The difference between true altitude and pressure altitude
must be clearly understood. True altitude means
the vertical height of the glider above MSL. True altitude
is displayed on the altimeter when the altimeter is
adjusted to the local atmospheric pressure setting.
For example, if the local altimeter setting is 30.12 in.
Hg., and the altimeter is adjusted to this value, the
altimeter indicates exact height above sea level.
However, this does not reflect conditions found at this
height under standard conditions. Since the altimeter
setting is more than 29.92 in. Hg., the air in this example
has a higher pressure, and is more compressed
indicative of the air found at a lower altitude.
Therefore, the pressure altitude is lower than the actual
height above MSL.
To calculate pressure altitude without the use of an
altimeter, remember that the pressure decreases
approximately 1 inch of mercury for every 1,000-foot
increase in altitude. For example, if the current local
altimeter setting at a 4,000-foot elevation were 30.42,
the pressure altitude would be 3,500 feet. (30.42 -
29.92 = .50 in. Hg. x 1,000 feet = 500 feet. Subtracting
500 feet from 4,000 equals 3,500 feet).
The four factors that affect density altitude the most are
atmospheric pressure, altitude, temperature, and the
moisture content of the air.
ATMOSPHERIC PRESSURE
Due to changing weather conditions, atmospheric pressure
at a given location changes from day to day. When
barometric pressure drops, air density decreases. The
reduced density of the air results in an increase in density
altitude and decreased glider performance. This
reduces takeoff and climb performance and increases
the length of runway needed for landing.
When barometric pressure rises, air density increases.
The greater density of the air results in lower density
altitude. Thus, takeoff and climb performance improves,
and the length of runway needed for landing decreases.
ALTITUDE
As altitude increases, air density decreases. At altitude,
the atmospheric pressure that acts on a given volume
of air is less, allowing the air molecules to space themselves
further apart. The result is that a given volume
of air at high altitude contains fewer air molecules than
the same volume of air at lower altitude. As altitude
increases, density altitude increases, and glider takeoff
and climb performance is reduced.
5-2
TEMPERATURE
Temperature changes have a large affect on density altitude.
When air is heated, it expands and the molecules
move farther apart, creating less dense air. Takeoff and
climb performance is reduced, while the length of runway
required for landing is increased.
The effects are different when the air is cool. When aircools,
the molecules move closer together, creating
denser air. Takeoff and climb performance improves,
and the length of runway required for landing
decreases.
The effect of temperature on density altitude can be
very great. High temperatures cause even low elevations
to have high-density altitudes, resulting in
reduced takeoff and climb performance. Very cold temperatures,
on the other hand, can result in density altitudes
that are far below those at sea level. In this dense,
cold air, takeoff and climb performance is enhanced
considerably.
MOISTURE
The water vapor content of the air affects air density.
Water vapor molecules, consisting of two hydrogen
atoms and one oxygen atom, have a relatively low
molecular weight. Water vapor molecules in the atmosphere
displace gas molecules with higher molecular
weights. Therefore, as the water vapor content of the
air increases, the air becomes less dense. The result is
increased density altitude and decreased takeoff and
climb performance.
Relative humidity refers to the amount of water vapor
contained in the atmosphere. It is expressed as a percentage
of the maximum amount of water vapor the air
can hold. Perfectly dry air (air that contains no water
vapor) has a relative humidity of 0 percent, while saturated
air (air that cannot hold any more water vapor)
has a relative humidity of 100 percent.
The amount of water vapor that an airmass can sustain
is affected by temperature. Cold air can hold a relatively
small amount of water as vapor; warm air can
hold much more. Increasing the temperature of an airmass
by 20°F doubles the amount of water the airmass
can hold as water vapor. Increasing the temperature of
an airmass by 40°F quadruples the amount of water the
airmass can hold. Increasing the temperature of an airmass
　
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本文鏈接地址：Glider Flying Handbook(44)