66 THE HEAT BUDGET OF THE OCEANS 



average level of pans used on shipboard and a level a few centimeters 

 above the sea surface, Wtist arrived at the conclusion that the measured 

 values had to be multiplied by 0.53 in order to represent the evaporation 

 from the sea surface. 



In computing the evaporation on the basis of the heat balance, one 

 has to start out from equation (IV, 2) (p. 50). Introducing the 

 ratio, R = Qh/Qe, putting Qs — Qb = Qr, and taking into account that 

 the evaporation, E, is obtained in centimeters by dividing Qe by the 

 latent heat of vaporization, L, one obtains 



In this form the equation representing the heat balance has found wide 

 application for computation of evaporation. The result gives the 

 evaporation in centimeters during the time intervals to which the values 

 Qr, and so on, apply, provided these are expressed in gram calories. 



Several attempts have been made to establish empirical formulas by 

 which the evaporation from the sea could be computed from meteor- 

 ological data and knowledge of the sea-surface temperature. It has been 

 assumed that the evaporation could be related to the difference in vapor 

 pressure at the very sea surface and the vapor pressure in the air as 

 observed on shipboard, Cw — ea, and to the wind velocity, Wa- Various 

 formulas of the type E = f[(e^a — Ca) Wa] have been proposed, but no 

 consistent results have been obtained because each formula has been 

 developed in order to represent some specific set of very uncertain observa- 

 tions. Another approach was suggested by Sverdrup, who, on the basis 

 of results in fluid mechanics as to the turbulence of the air over a rough 

 surface, established a formula for the evaporation, using, in part, con- 

 stants that had been determined by laboratory experiments, and, in part, 

 constants that were obtained from the character of the variation of vapor 

 pressure with increasing height above the sea surface. Somewhat 

 similar but more complicated formulas have been derived by Millar 

 and by Montgomery. 



These investigations indicate that at wind velocities below 4 to 

 5 m/sec (10 miles per hour) the evaporation is relatively small, partly 

 because at low wind velocities and stable stratification of the air the 

 sea surface has the character of a hydrodynamically smooth surface 

 (p. 120), and partly because at higher wind velocities the evaporation is 

 greatly increased on account of spray. Furthermore, it is found that at 

 moderate and high wind velocities the evaporation can be written 



E = k{e^ - ea)Wa, (IV, 11) 



where the factor k is nearly a constant, the numerical value of which may 

 be determined when the character of the variation of vapor content with 



