Tables 98 and 99 355 



REDUCTION OF PSYCHROMETRIC DATA 



Two distinct theories of the psychrometer have been advanced, the convection theory of 

 August and the diffusion theory of Maxwell. Both predict the same (correct) form of 

 the humidity-temperature relationship, but neither gives a correct value for the propor- 

 tionality constants because of their physical incompleteness. Arnold 1 has presented an 

 analysis which embodies the effects of convection, diffusion, and radiation. 



In agreement with theory, many empirical workers have verified, for the system water- 

 air, the validity in restricted range of air velocities of the expression 



where : 



P(t-t') 



(1) 



* = air temperature, 



t' = wet-bulb temperature, 



p = barometric pressure, 



e = vapor pressure in the air, 

 e = saturation vapor pressure at temperature *', 



A = proportionality factor which for a given ventilation velocity and instrument 

 varies slightly with *'. 



From an examination of a large number of observations, Ferrel ' obtained for centigrade 

 temperatures 



A = 0.000660(1 + 0.00115*') (2) 



Subsequent investigations by Brooks and Allen 8 and others have resulted in slightly dif- 

 ferent constants, but this difference is so slight that there is no advantage in altering the 

 conventional formula of Ferrel and thereby destroying consistency in psychrometric ob- 

 servations. Although Ferrel's observations included only a very limited number of cases 

 with wet-bulb temperatures below freezing and these in the neighborhood of °C, they 

 indicated the constants that applied to liquid-covered wet bulbs also applied to ice-covered. 

 Brooks and Allen have substantiated his results provided the wet-bulb is covered with a 

 frozen wick* In experiments using a thin layer of ice directly on the bulb, the constants 

 of Ferrel were invalid and should be multiplied by the ratio of the latent heat of evapora- 

 tion of water to that of ice which is 0.882. 



Psychrometric observations. — In making psychrometric observations, Resolution 145, 

 International Meteorological Organization, Twelfth Conference of Directors (Washington, 

 1947) recommends that air be drawn past the bulbs at a rate not less than 4 meters per 

 second and not greater than 10 meters per second, if the thermometers are of the types 

 ordinarily used at meteorological stations. Resolution 145 also emphasizes the necessity for 

 using clean, washed wicks and pure water, and draws attention to the fact that supercooled 

 water may exist on the bulb at temperatures well below °C, and that if this is not 

 noticed by the observer, very serious errors will occur. The freezing can be initiated by 

 touching the bulb with clean snow or ice, a pencil, or other object. For a discussion on 

 low-temperature psychometry see Wile. 5 



While concise, Tables 98 and 99 are not best suited for routine use in reducing psychro- 

 metric observations. Where many reductions are to be made, access should be obtained 

 to one of the many tables,*' T nomograms, 8 and slide rules B constructed for this purpose. 



1 Arnold, J. Howard, The theory of the psychrometer, Physics, vol. 4, pp. 2SS-262, 334-340, 1933. 

 See also Dropkin, David, The deviation of the actual wet-bulb temperature from the temperature of 



adiabatic saturation, Cornell Univ. Eng. Exp. Stat. Bull. 23, 1936; Effect of radiation on psy- 

 chrometry readings, Cornell Univ. Eng. Exp. Stat. Bull. 26, 1939. 



2 Ferrel, Wm, Ann. Rep. Chief Signal Officer, 1886, app. 24, pp. 233-2S9. 



8 Brooks, Donald B., and Allen, H. H., Journ. Washington Acad. Sci., pp. 121-134, 1933. 



* Note. — On the basis of certain other investigations, some European psychrometer tables use as 

 the value of A for ice-covered bulbs 0.882 times the value of A for water, e.g., see Trabert, W., 

 Jelineks Psychrometer-Tafeln, Leipzig, 1911. 



5 Wile, D. D., Psychrometry in the frost zone, Refrig. Eng., October 1944. 



8 Marvin, C. F., Psychrometric tables, U. S. Dep. Commerce, Weather Bur., No. 235, 1941. 



' Goodman, William, Air conditioning analysis with psychrometric charts and tables, 455 pp., Mac- 

 millan, 1943. 



8 Brooks, D. B., Psychrometric charts for high and low pressures, Nat. Bur. Stand. Misc. Publ. 

 M 146, 1935. 



9 U. S. Weather Bur. Psychrometric calculator, Nos. 1183 and 1184. 



(continued) 



SMITHSONIAN METEOROLOGICAL TABLES 



