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<br />0014 77' <br /> <br />. . <br /> <br />Air and dewpoint temperatures are observed hourly at about 750 <br />stations on the land surface in the United States. About 300 addi- <br />tional stations are equipped with recording instruments which provide <br />weekly charts of temperature and relative humidity. Upper-air <br />humidity is observed by radiosonde at about 60 stations in the United <br />states, usually t,vice daily. <br /> <br />The standard non-recording instrument for measuring humidity, the <br />psychrometer, consists of two identical thermometers, of which one has <br />its bulb covered with wet muslin. After adequate ventilation, by <br />whirling or by" fan, the "dry bulb" and wet bulb" temperatures are read. <br />Then, the relative humidity, dewpoint temperature, or vapor pressure are <br />determined by means of psychrometric tables or special slide rules. <br /> <br />The more common recording hygrometer, or hygrograph, is actuated by <br />a human-hair or other element which expands and contracts in proportion <br />to the relative humidity of the air. These physical changes move a pen <br />arm over a' chart on a drum .Thich is rotated by clockwork to record a <br />continuous curve. Another type of recording hygrometer depends on change <br />of electrical properties of the sensing element due to absorption of <br />moisture; ~hese lend themselves to use in radiosondes and in remote <br />installations from which the measurements are transmitted electrically. <br /> <br />Hygrothermographs are, as the name implies, combination hygrographs <br />and thermographs. Usually they consist of a hair element and a bi-meta~ic <br />thermometer, both activating pens which trace relative humidity and temper- <br />ature on a'chart on a rotating drum. <br /> <br />Reliability of observations <br /> <br />Instrumental errors <br /> <br />The measurement of atmospheric water vapor is one of the least <br />accurate i~strumental procedures in meteorology. The psychrometer, <br />besides being subject to the same observational errors as the thermometer, <br />is subject to errors due to improper wetting of the w:i<::k, ,.nadequate <br />ventilation, or impure water. The speed with which hair hygrometers and <br />hygrograph$ respond to changes in humidity varies with temperature. The <br />response is very slow at low temperatures, the lag becoming almost infinite <br />at _400 F. Such lag may increase with age of the instrument, as the\ <br />mechanical parts wear. In consequence, the recorded values may be too <br />large when' humidity is diminishing and too small when humidity is increa$ing. <br />Because ertor from this cause can be substantial when the relative humidity <br />changes ra~idly, as in upper-air soundings, use of hair elements in radi~- <br />sondes has been discontinued. <br /> <br />i <br />t': ;\ <br /> <br /> <br />V.2 <br />