For an acceptable result with slightly less precision,you may use the density altitude diagram (fig. 1-41) toobtain density altitude to the nearest 200 feet. Thisdiagram also ignores the effect of humidity on densityaltitude. Enter the bottom of the diagram with your airtemperature and proceed vertically to the intersection ofthe pressure altitude line, then horizontally to the leftside of the diagram to find the density altitude. The lightdashed line shows an example using 22°C and apressure altitude of 10 feet, resulting in a densityaltitude of about 1,000 feet.You may interpolate for more precise values, butthis precision isn’t often necessary for most densityaltitude calculations. (A quick method of determiningstandard temperatures in degrees Celsius for all levelsup to 35,000 feet is to double the altitude in thousands offeet, subtract 15, and change the sign.)SPECIFIC HUMIDITYSpecific humidity is the mass of water vapor presentin a unit mass of air. Where temperatures are high andrainfall is excessive, the specific humidity of the airreaches high proportions. Accurate information isrequired to determine the proper amount of horsepowerneeded for the takeoff roll.Fog and humidity affect the performance ofaircraft. During takeoff, two things are done tocompensate for their effect on takeoff performance.First, since humid air is less dense than dry air, theallowable takeoff gross weight is generally reduced foroperations in areas that are consistently humid. Second,because power output is decreased by humidity, pilotsmust compensate for the power loss.Your mainresponsibility as an Aerographer’s Mate is to ensure thatthe pilot has accurate information. Pilots may requesthumidity values as either relative humidity (discussedin the previous section) or specific humidity.Figure 1-41.—Density altitude diagram.1-64
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