THE HEAT BUDGET OF THE OCEANS 67 



height is better known. Introducing average annual values of the vapor 

 pressure (in millibars) and average annual wind velocity (in meters per 

 second), one obtains the evaporation in centimeters per j^ear by putting 

 k = 3.6, but this numerical value depends upon the heights at which the 

 vapor pressure in the air and the wind velocity are measured, and should 

 therefore be used with caution. 



Average Annual Evaporation from the Oceans. On the basis 

 of pan measurements conducted in different parts of the ocean, Wtist 

 found that the average evaporation from all oceans amounts to 93 cm 

 per year, and he considers this value correct to within 10 or 15 per cent. 

 W. Schmidt computed the evaporation for E by means of equation 

 (IV, 10), in which the terms (>„ and Q^ can be omitted when considering 

 the oceans as a whole. Schmidt introduced a high value of R, and on 

 the basis of the available data as to incoming radiation and back radiation 

 he found a total evaporation of 76 cm a year. A revision based on more 

 recent measurements of radiation (Mosby) and use of i? = 0.1 resulted 

 in a value of 106 cm a year. The latter value represents an upper limit 

 and may be 10 to 15 per cent too high, wherefore it appears that Wiist's 

 result is nearly correct. 



It is of interest in this connection to give some figures regarding the 

 relation between evaporation and precipitation over the oceans, the land 

 areas, and the whole earth. According to Wust the total evaporation 

 from the oceans amounts to 334,000 km^/year, of which 297,000 km^ 

 returns to the sea in the form of precipitation, and the difference, 37,000 

 km^, must be supplied by run-off. The total amount of precipitation 

 falling on the land is 99,000 km^, of which amount a little over one third, 

 37,000 km^, is supplied by evaporation from the oceans and 62,000 km^ 

 is supplied by evaporation from inland water areas or directly from the 

 moist soil. For the sake of comparison it may be mentioned that the 

 maximum capacity of Lake Mead above Boulder Dam is 45 km^. 



Evaporation in Different Latitudes. From pan observations 

 at sea, Wlist has derived average values of the evaporation from the 

 different oceans in different latitudes (table 10, p. 72). By means of the 

 energy equation, one can compute similar annual values, assuming that 

 the net transport of heat by ocean currents can be neglected. Such a 

 computation has been carried out for the Atlantic Ocean, making use of 

 Kimball's data as to the incoming radiation of the observed temperatures 

 and humidities for determining effective back radiation. In fig. 13 

 are shown Wiist's values for the annual evaporation between latitudes 

 50° N and 50° S in the Atlantic Ocean and the corresponding values as 

 derived from the energy equation. The low evaporation in the equatorial 

 regions which both methods show can be ascribed to the higher relative 

 humidities and the lower wind velocities of that area, if one considers 

 the processes of evaporation, or it can be ascribed to the effect of the 



