SPECIFIC HUMIDITY

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. 1-64