For an acceptable result with slightly less precision,
you may use the density altitude diagram (fig. 1-41) to
obtain density altitude to the nearest 200 feet. This
diagram also ignores the effect of humidity on density
altitude. Enter the bottom of the diagram with your air
temperature and proceed vertically to the intersection of
the pressure altitude line, then horizontally to the left
side of the diagram to find the density altitude. The light
dashed line shows an example using 22°C and a
pressure altitude of 10 feet, resulting in a density
altitude of about 1,000 feet.
You may interpolate for more precise values, but
this precision isn’t often necessary for most density
altitude calculations. (A quick method of determining
standard temperatures in degrees Celsius for all levels
up to 35,000 feet is to double the altitude in thousands of
feet, subtract 15, and change the sign.)
SPECIFIC HUMIDITY
Specific humidity is the mass of water vapor present
in a unit mass of air. Where temperatures are high and
rainfall is excessive, the specific humidity of the air
reaches high proportions. Accurate information is
required to determine the proper amount of horsepower
needed for the takeoff roll.
Fog and humidity affect the performance of
aircraft. During takeoff, two things are done to
compensate for their effect on takeoff performance.
First, since humid air is less dense than dry air, the
allowable takeoff gross weight is generally reduced for
operations in areas that are consistently humid. Second,
because power output is decreased by humidity, pilots
must compensate for the power loss.
Your main
responsibility as an Aerographer’s Mate is to ensure that
the pilot has accurate information. Pilots may request
humidity values as either relative humidity (discussed
in the previous section) or specific humidity.
Figure 1-41.—Density altitude diagram.
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