286 



TEMPERATURE AND HUMIDITY 



that he found the relationship between Nus- 

 selt's and Reynolds' numbers broke down. 

 Hence, this velocity may be tentatively as- 

 sumed to be the upper limit of appHcation 

 for these equations. 



During the last few years Plummer (33) 

 has made an extension of Winslow's data 

 (41) by reducing the characteristic dimen- 

 sions of the human body to a cyhnder 7 cm. 

 in diameter. The curve given below is his 

 plot of the convection loss of a cylinder for 

 various wind velocities. 



Two physiological measurements and two 

 physical measurements are required to com- 

 pute convection losses. The physiological 

 measurements are: (a) the skin or surface 

 temperature, and (b) the characteristic di- 

 mension of the body. The physical meas- 

 urements, which are relatively simple to 

 make are: (c) the air temperature, and (d) 

 the velocity. The measurements of skin 

 temperature will be difficult to make, since 

 the thermometers or other measuring ele- 

 ments must not affect the skin's being cooled 

 by the moving air, and they must not them- 

 selves be affected by the air stream. The 

 measurement of the characteristic dimension 

 will require a series of measurements of con- 

 vection loss by difference as affected by air 

 velocity. 



Evaporation 



The continual loss of weight from the body 

 has been of interest to physiologists for about 

 300 years, and the mass of experimental data 

 that has been gathered by many workers 

 under varied conditions has established the 

 importance of this loss as a factor in the heat 

 regulation of the body. Soderstrom and 

 DuBois (37) have determined that about 25 

 percent of the metabolic heat is carried away 

 from the body by the water evaporated from 

 the skin and lungs. The importance of the 

 physiological control of the water evaporated 

 from the skin has been repeatedly pointed 

 out by Rubner, Hill, Kuno, Newburgh, and 

 many others, and in the last twenty years 

 the physical laws concerned with the loss of 



heat by evaporation from the body have 

 been carefully worked out by Biittner (9), 

 Gagge (16), and others. Evaporation takes 

 place at the skin surface, the hquid sweat 

 passing into vapor at about skin temperature 

 and finally passing into the environment to 

 cool and to expand to the saturation of the 

 environment. 



Although moisture is lost from both the 

 skin and lungs , from the standpoint of heat loss 

 the former is the more important. Winslow 

 et al. (43) measured the heat loss from the 

 lungs of their subjects and obtained a value 

 of 7-8 kg. cal./hr., about 10 percent of the 

 total heat loss in the neutral temperature 

 zone. Sweating from the skin is the impor- 

 tant means of losing heat at elevated envi- 

 ronmental temperatures. 



Biittner (9) pointed out the importance 

 of the amount of skin which can be con- 

 sidered wet, and of the vapor pressure dif- 

 ference between saturated vapor at skin 

 temperature and the vapor pressure of the en- 

 vironment. Gagge (15) has given a formula 

 for heat loss by vaporization which is similar 

 to an earlier one by Biittner (9) : 



(12) Hv = (c^n)A{Es - RHEA)kg.ca\./hT., 



or 



(12a) (com) = 



H, 



A{Es - RHE^y 



where co = fraction of body area that is com- 

 pletely wet, M = proportionality factor con- 

 taining the vaporization constant and the 

 factors which depend on air velocity and di- 

 rection, Hv = heat loss by vaporization, 

 A = total body area, and RH = relative 

 humidity. 



It is seen from Gagge 's equation that when 

 the vapor pressure {RH X Ea) of the air 

 equals that of the skin, no vaporization will 

 take place, and if the air vapor pressure 

 should be greater than that on the skin, 

 moisture will condense on the skin and the 

 body will gain heat. 



Hardy and DuBois (22), and Winslow 

 et al. (43) have shown that within the zone 



