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<br />0014 78 <br /> <br />Instrumental error due to effects of temperature may remain even <br />after stahdard measures are taken to correct the indicated readings. <br />This applies in particular to the hair hygrometer. <br /> <br />In recent years, upper-air soundings have been w.ith electrical <br />hygrometers. These instruments also may develop some lag as they age, <br />as the electrical properties of the sensing element change. Ordinarily, <br />however, their error due to lag is inconsequential. <br /> <br />Environmental errors <br /> <br />As has been implied, a measurement of humidity represents, strictly, <br />an atmosptleric property at only one point in space at one instant of <br />time. Thll atmosphere can vary w.idely and abruptly in this property frOM <br />one place to another (geographic), from one altitude above land surface <br />to another, and from one time to another. Consequently, substantial <br />error can'result if data on humidity are applied uncritically to l$nd- <br />surface e~vironments, altitudes, or times that differ from thOse which <br />the data represent. <br /> <br />Uses of data on humidity <br /> <br />The rlltio of the water precipitated in a given storm to all the <br />water vap~rpresent in the atmosphere at that place and time--that is, <br />to the prtcipitable water--is used in deriving estimates of probable <br />maximum precipitation. Precipitable water is used also in preparing <br />quantitat~ve forecasts of preoipitation. <br /> <br />The v~rt;!.cal gradient of vapor pressure, determined by observations' <br />at two altitudes, has been used to estimate evaporation. Almost all <br />formulas for estimating evaporation from a water or snow 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 e:kceeds that of a snow surface, condensation, hence melting, <br />will take place instead of evaporation. <br /> <br />In co~uting snow melt resulting from rainfall, wet.bulb temperature <br />of the aiT is substituted for actual temperature of the rain. <br /> <br />v.) <br /> <br /> <br />.,"t <br />