68 



THE HEAT BUDGET OF THE OCEANS 



prevailing cloudiness, if one considers the energy relations. The great 

 evaporations in the areas of the strong trade winds appear clearly, but in 

 the Southern Hemisphere the observations give the highest values of the 

 evaporation nearer to the Equator than do the computations. The 

 discrepancy may be due to the fact that in the course of a year the wind 

 systems change their distances from the Equator and that the observa- 

 tions have not been distributed evenly over the year. The energy 

 equation has also been used by McEwen for computing values of evapora- 

 tion over the eastern Pacific Ocean between latitudes 20° N and 50° N. 

 His figures agree with those obtained by Wiist for the same latitudes. 



It appears that the average annual values of the evaporation in 

 different latitudes are well established, but the evaporation also varies 

 from the eastern to the western parts of the oceans and with the seasons. 



40^ N 3or 20° 



LATITUDE 

 \(f 0° 10° 



ZCf 30° S40° 



Fig. 13. Annual evaporation from the Atlantic 

 Ocean between lat. 50°N and 50°S. The thin curve is 

 based on observations (Wiist) and the heavy curve on 

 computations, using the energy equation. 



These variations, which are closely related to the ocean currents, are of 

 great importance to the circulation of the atmosphere, because the 

 supply of water vapor which later on condenses and gives off its latent 

 heat represents a large portion of the supply of energy. Approximate' 

 values of the evaporation from different parts of the oceans and in differ- 

 ent seasons can be found by means of the method proposed by Sverdrup, 

 and will be dealt with after the ocean currents have been discussed. 



Annual Variation of Evaporation. The character of the annual 

 variation of evaporation can be examined by means of the energy equation 



Qa = ^',(1 + R) = Qr - Qv — Qf}- 



(IV, 12) 



The quantity q^ can be computed if the annual variation of temperature, 

 due to processes of heating and cooling is known at all depths where such 

 annual variations occur. The annual variation of temperature at the 

 surface has been examined, but only few data are available from sub- 

 surface depths, the most reliable being those that have been compiled by 

 Helland-Hansen from an area in the eastern North Atlantic with its 



