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<br /> <br />" <br /> <br />",'," <br /> <br />';'18. <br /> <br />Hm~IDITY(ATYjOSPHERrc WATER VAPOR) <br /> <br />Location and installation of measuring equipment <br /> <br />The quantity of water vapor in the atmosphere, or,the degree of <br />humidity, varies with (1) altitude abOVe the land s\Ir:~a()e; (2) type of <br />surface--th,at is, land or water; vegetated or barren; (3) hour of the day, <br />with attendant changes in sunshine, air tempel:'ature, and baX'ometric pres- <br />sure; and (4) eharaderistics of the air mass that surroundS the poj,nt <br />in question, In a cloud or fog the water-vapor content may well he <br />greater than in the air elsewhere. Usually; hUmidity diminishes rapid:!,y <br />crith distance from a laks or seashore. Humidity tends to be gl'eater ir) <br />forested areas than oVer barren soH, even when both are under the influ- <br />ence of the same air mass. Thus, a measurement of humidity, e$,pecially <br />one marJe a't low altitude above the land surface, commonly represents only <br />its immediate vicinity. <br /> <br />At the land surface, instruments to measure humidity generally are <br />exposed in the same shelter as the the~mometers, and hence are subjeot <br />to the same criteria of location and exposure. Upper-air measurements <br />are transmitted from sensing elements of a radiosonde carried aloft by <br />balloon. These elements are well ventilate,d and protected frOm direct <br />sblar radiation and from precipitation. <br /> <br />Measurements and measuring eguipment <br /> <br />The amount of water vapor in the air, or degree of humidity, may be <br />expressed in various terms, according to intended use of the data. The <br />terms defined in this section include only those which the hydrologist is <br />likely to find in hydrometeorologicalor hydrologic literature. <br /> <br />"Actual vapor pressure" is the pat'tial pressure of the wa-ter vapor <br />present in the sample of air under consideration. "Saturation vapor <br />pressure" is the maximUlll vapor preSs1),re that can ocour at the existing <br />temperature, "Relative humidity" is the !'atio of actual vapOr> pressure <br />to saturated vapor pressure, exp:1'esse~ as a percentage. "Dewpoint <br />temperature" is that temperature to which a parcel of air must be cooled <br />at constant pressure and water~vapor content for saturation to occur. <br />"Precipitable water" is tjJe depth ,of water th'lt would result over a unit <br />area .if all tbe water vapor in a G'illlumn of, air ofuni'G cross section were <br />condensed and precipitated. Its Gompu:tation is based on the water-vapor <br />content at all levels in the column. In a,aense the term is a misnomer <br />as there is no natural process that will completely remove all water <br />vapor from the air. <br /> <br />Air and dewpoint temperatures at'e observe~'bourly at about 750 sta- <br />tions on the land surface in theun;l.tedS:(;ates. About 300 additional <br />stations are equipped with recording instruments whieb provide weekly <br />charts of temperature and relative humidit1. Upper-air humidity is <br />observed by radiosonde at about 60 stations in the United states, usually <br />twice daHy. <br />