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V.l 0014 76 V. HUMIDITY (ATMOSPHERIC WATER VAPOR) Looation and installation of measuring equipment The quantity of water vapor in the atmosphere, or the degree of humidity, varies with (1) altitude above the land surface; (2) type of surface--that is, land or water, vegetated or barren; (3) hour of the day, with attendant changes in sunshine, air temperature, and barometric. pressure; and (4) characteristics of the air mass that surrounds the point in question. In a cloud or fog the water-vapor content may well be greater than in the air elsewhere. Usually, humidity diminishes rapidly with distance from a lake or seashore. Humidity tends to be greater in forested areas than over barren soil, even when both are under the influence of the same air mass. Thus, a measurement of humidity, especially one made at low altitude above the land surface, commonly represents only its immediate vicinity. At the land surface, instruments to measure humidity generally are exposed in the same shelter as the thermometers, and hence are .subject to the sante criteria of location and exposure. Upper-air mea.surements are transmitted from sensing elements of a radiosonde carried alGft.b,y balloon. 'I'hese elements are well ventilated and protected from direct solar radiation and from precipitation. Measurements and measuring equipment The am:ount of water vapor in the air, or degree of humidity, may be expressed in various terms, according to intended use of the data. The terms defina:l in this section include only those which the b,ydrologist is likely to find in hydrometeorological or hydrologic literature. "Actual vapor pressure" is the partial pressure of the water vapor present in the sample of air under consideration. "Saturation vapor pressure" is the maximum vapor pressure that can occur at the existing temperature. "Relative humidity" is the ratio of actual vapor pressure to saturated vapor pressure, expressed as a percentage. "DeWIlOint temperatUJ:'e" is that temperature to which a parcel of air must be cooled at constant pressure and water-vapor content for saturation to occur. "Precipitable water" is the depth of water that would result over a unit area if all the water vapor in a column of air of unit cross section were condensed and precipitated. Its computation is based on the 1l'ater-vapor content at all levels in the column. In a sense the term is a niisnomer as there is no natural process that will completely rElmove all water vapor from the air.