62 THE HEAT BUDGET OF THE OCEANS 



The sea surface must be warmer than the air at a small distance above 

 the' surface if heat shall be conducted from the sea to the air. When 

 such conditions prevail, the air is heated from below, the stratification of 

 the air becomes unstable, and the turbulence of the air becomes intense. 

 If the sea surface is very much warmer than the air, as may be the case 

 when cold continental air flows out over the sea in winter, the heating 

 from below may be so intense that rapid convection currents develop, 

 leading to such violent atmospheric disturbances as thunderstorms. The 

 point which is emphasized is that an appreciable conduction of heat 

 from the sea to the atmosphere takes place when the sea surface is 

 warmer than the air. One might assume that, vice versa, an appreciable 

 amount of heat would be conducted to the sea surface when warmer air 

 flows over a cold sea, but this is not the case, because under such condi- 

 tions the air is cooled from below, the stratification of the air becomes 

 stable, and the turbulence, and consequently the eddy conductivity of 

 the air, is greatly reduced. 



It has been found (p. 75) that on an average the sea surface is 

 slightly warmer than the overlying air and therefore loses heat by conduc- 

 tion. So far, no detailed studies have been made, but Angstrom has 

 estimated that only about 10 per cent of the total heat surplus is given 

 off to the atmosphere by conduction and that 90 per cent is used for 

 evaporation. Other estimates indicate that these figures are approxi- 

 mately correct (p. 64). Thus, evaporation is of much greater impor- 

 tance to the heat balance of the oceans than is the transfer of sensible 

 heat. 



Evaporation from the Sea 



The Process of Evaporation. The vapor tension at a flat surface 

 of pure water depends on the temperature of the water. The salinity 

 decreases the tension slightly, the empirical relation between vapor 

 tension and salinity being (p. 15) 



e^ = ed(l - 0.0053 S), 



where Cw is the vapor tension over sea water, Cd is the vapor tension over 

 distilled water of the same temperature, and S is the salinity in parts per 

 thousand. In the open ocean Cw = 0.98(?d, approximately. 



In discussing the process of evaporation, it is more rational to con- 

 sider not the vapor pressure but the specific humidity, q — that is, the mass 

 of water vapor per unit mass of moist air. The amount of water vapor, 

 F, which per second is transported upward through a surface of cross 

 section one square centimeter is, then, —^edq/dz, where //« is the eddy 

 diffusivity, which in the air equals the eddy viscosity, and —dq/dz is 

 the vertical gradient of the specific humidity, which is positive when the 

 specific humidity decreases with height. If the vapor pressure, e, is 



