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In this article we will discuss about the meaning and variation of absolute humidity.
The actual amount of water vapour in a given volume of air, that is, the weight per volume, such as pounds per 1,000 cubic feet, is called the absolute humidity. A direct relationship exists among the dew point, the vapour pressure, and the absolute humidity because, at constant atmospheric pressure, each of these depends only on the actual amount of water vapour in the air. At saturation, the dew point is the same as the temperature, the vapour pressure is the saturation vapour pressure, and the absolute humidity is the saturation absolute humidity.
Table Dew Point, Vapour Pressure, and Absolute Humidity show the relationship among these three measures of atmospheric moisture. Saturation values of vapour pressure and absolute humidity can be obtained by entering temperature instead of dew point in the first column. Because of these relationships, the temperature of the dew point is a convenient unit of measure for moisture. Air temperature and dew point accurately define atmospheric moisture at any time or place.
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Absolute humidity varies in space and time for several reasons; however, relative humidity does not necessarily change in the same manner, because relative humidity is very dependent upon air temperature. The temperature effect frequently overrides the absolute humidity effect; therefore, relative humidity usually varies inversely with temperature.
Variations in Absolute Humidity:
The actual amount of moisture in air will vary from one air mass to another, and even within an air mass there will be continuing variations in time and space.
The moisture contents of air masses are basically related to their regions of origin. Air masses originating in continental areas are relatively dry. Those coming from large bodies of water, such as the Atlantic or the Gulf of Mexico are moist. Those from the Pacific are moist or moderately moist. As these maritime air masses invade the continent, land stations will observe abrupt rises in absolute humidity. As any air mass traverses areas different from its source region, gradual changes take place as evaporation, transpiration, condensation, and precipitation add or subtract moisture.
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Through a deep layer within an air mass, the absolute humidity, like the temperature, usually decreases with height. There are several reasons for this distribution. First, moisture is added to the atmosphere from the surface and is carried upward by convection and upslope and up valley winds.
Second, when air is lifted, the water vapour, as well as the air, expands proportionately so that the moisture in any given volume becomes less and less. Thus, the absolute humidity decreases as the air is lifted. Third, since temperature usually decreases upward, the capacity for air to hold moisture decreases upward. Finally, the precipitation process removes condensed moisture from higher levels in the atmosphere and deposits it at the surface.
The normal pattern of decrease of moisture with altitude may be altered occasionally when horizontal flow at intermediate levels aloft brings in moist air. Such flow is responsible for much of the summer thunderstorm activity over large parts of the West. Extremely low absolute humidity is found in subsiding air aloft. This dry air originates near the top of the troposphere and slowly sinks to lower levels. If it reaches the ground, or is mixed downward, it may produce acutely low humidity near the surface and an abrupt increase in fire danger.
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If we consider only a very shallow layer of air near the surface, we find that the vertical variation of absolute humidity with height will change during each 24-hour period as conditions favouring evaporation alternate with conditions favouring condensation. During clear days, moisture usually is added to the air by evaporation from warm surfaces; therefore, the absolute humidity decreases upward. At night, moisture is usually taken from the air near the surface by condensation on cold surfaces and absorption by cold soil and other substances; thus, the absolute humidity may increase upward through a very shallow layer.