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<br /> <br />...20- <br /> <br />Environmental errors <br /> <br />As has been implied, a measurement of h1.lIi1:l,dity represents, strictJ.:y, <br />an atmospheric property at only one po:!,nt in space at one instant of <br />time. The atmosphere can vary widely and abruptlY ,in this property frcm <br />cine place to another (geographic), from one aJ,t:!, tude above land surface <br />to another, and from one time to another. Consequently, substantial <br />error can result if data on humidity' are applied unc~itically to land~ <br />surface environments , altitudes, or times that differ from those which <br />the data represent. <br /> <br />Uses Of data on humidity <br /> <br />The ratio of the water precipitated in a given storm to all the <br />water vapor present ill the atmosphere at that place /ilnd time~..that is, <br />to the precipitable water-_is used in deriving estimates of probable <br />maximum precipitation. Precipitable water is used also in preparing <br />quantitative forecasts of precipitation. <br /> <br />The vertical gradient of vapor pressure, determined by observations <br />at two altitudes, has been used to estimate evaporaticn. Almost all <br />formulas for estimating evaporation from a water or sncw surface show <br />evaporation to be proportional to the difference between the vapor <br />pressure of the water and that of the air. When the vapor pressure of <br />the air exceeds thst of a snow surfaCe, condensation, hence melting, <br />will take place instead of evaporation. <br /> <br />In oomputing snowmelt resulting from raini'all, wet~oulb temper"ture <br />of the air is substituted for actual temperature of the rilin. <br />