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Water Content of the Air

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Water Content of the Air

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 Aerographers’s Mate is to ensure that the pilot has accurate information.

VAPOR PRESSURE. —Vapor pressure is that portion of the atmospheric pressure that is exerted by water vapor in the atmosphere. It is expressed in inches and tenths of an inch of mercury (Hg). The dew point for a given condition depends on the amount of water vapor present, so a direct relationship exists between vapor pressure and the dew point.

SPECIFIC HUMIDITY. —The mass of water vapor present in a unit mass of air is known as specific humidity. The mass of the unit of air is considered to be a unit mass of moist air. Since the mass of a unit of dry air differs but little from the mass of a unit of moist air, the mixing ratio and specific humidity are nearly numerically equivalent.

Where temperatures are high and rainfall is excessive, the water vapor content of the air reaches high proportions. Accurate information is required to determine the proper amount of horsepower needed for the takeoff roll. Specific humidity and vapor pressure can be determined from the Density Altitude Computer.

Learning Objective: Define d-value and recognize how d-values are computed.

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