THE HEAT BUDGET OF THE OCEANS 63 



introduced, one obtains approximately 



„ 0.621 de .j^j. ^. 



F=-,.~j-j^. (IV, 6) 



where p is the atmospheric pressure. The heat needed for evaporation 

 at the surface is 



n T 0-^21 de , 



where L^ is the heat of vaporization at the temperature of the surface, ^. 



The variation of the vapor pressure with increasing height above the 

 sea surface is closely related to the character of the eddy diffusivity. 

 From analogy with experimental results in fluid mechanics it has been 

 concluded that, when the air is' in a normal state of turbulence, the eddy 

 diffusivity increases linearly with increasing distance from the sea surface, 

 except within a very thin layer close to the surface. The implication is 

 that, when a steady stage has been reached, the specific humidity must 

 be proportional to the logarithm of the distance from the surface, and 

 observations have to some extent confirmed this conclusion. The reser- 

 vation that w^as made when dealing with temperature observations near 

 the sea surface must be made in this case as well, because measurements 

 conducted from vessels are affected by a considerable disturbance of the 

 normal field. It appears, however, that for the proper interpretation of 

 observations it is necessary to assume that next to the sea surface a 

 layer of air of a thickness of a few^ millimeters exists within which the 

 transport of water vapor takes place by ordinary processes of diffusion. 

 Above this laj^er, the thickness of which varies with the wind, a transition 

 layer must be assumed, and finally there is a layer within which the eddy 

 diffusivity increases linearly wath the distance from the sea surface and 

 also increases w^ith the wind velocity. 



The ratio between the amounts of heat given off to the atmosphere as 

 sensible heat and used for evaporation is 



d^ dd- 



J. _Qh _ c^ V dz _ p dz , . 



dz dz 



The last expression is obtained by introducing Cp = 0.240 and L^ = 585. 

 Thus, the ratio R depends mainly upon the ratio between the temperature 

 and humidity gradients in the air at a short distance from the sea surface. 

 These gradients are difficult to measure but can be replaced approxi- 

 mately by the difference in temperature and vapor pressure at the sea 

 surface and the corresponding values in the air at a height of a few meters : 



R = 0.64 ^ ^-^^- (IV, 9) 



