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-19- 0;'1"] "51 ,:I ,~'- !..t' . The standard non-recording instrument ,for measuring humidity, the psychrometer, consists of two identical thermometers, of which one has its bulb covered, with wet muslin. After adequate ventilation, by whirling ,or by fan, the "dry bulb" ,and nwet bulb" temperatures are read. Then, the relative humidity, dewpoint temperature, or vapor pressure are determined by means of psychrometric tables or special slide rules. The more common recording hygrometer, or hygrograph, is actuated by a human-hair or other element which, expands and contracts in proportion to the relative humidity of the air. These physical changes move a pen arm over a chart on a drum which is rotated by clockwork to record a continuous curve. Another type of recording hygrometer depends on change of electrical properties of the sensing element due to absorption of moisture; these lend themselves to use in radiosondes and in remote installations from which the measurements are transimtted electrically. Hygrothermographs are, as ,the name implies, combination hygrographs and thermographs. Usually they consist of a hair element and a bi- metallic thermometer, both activating pens which trace relative humidity and 'temperature on a chart on a rotating drum. Reliability of observations Instrumental errors The measurement of atmospheric water vapor is one of the least accurate instrumental procedures in meteorology. The psychrometer, besides being subject to the same observational errors as the thermometer, is subject to errors due to improper wetting of the wick, inadequate ventilation, or impure water. The speed with which hair hygrometers and hygrographs respond to changes in humidity varies with temperature. The response is very slow at low temperatures, the lag becoming almost infi- nite at - 400 F. Such lag may increase with age of the instrument, as the mechanical parts wear. In consequence, the recorded values may be too large when humidity is diminishing and too small when humidity is increasing. Because error from this cause can be substantial when the relative humidity changes rapidly, as in upper-air soundings, use of hair elements in radiosondes has been discontinued, Instrumental error due to effects of temperature may remain even after standard measures are taken to correct the indicated readings. This applies in particular to the hair hygrometer. In recent years, upper-air soundings have been with electrical hygrometers. These instruments also may develop some lag as they age, as the electrical properties of the sensing element change. Ordinarily, however, their error due to lag is inconsequential.