Chapter VII 



Evaporation from the Surface of the 



Sea and the Water Budget of 



the Earth 



1. Introduction 



One of the most important problems with which both meteorology and oceano- 

 graphy is concerned is the water budget of the Earth. It can be assumed with a very 

 considerable degree of probability that the cycle through which the water passes is 

 closed. This follows, if a sufficiently long period is taken into consideration, from the 

 constancy of the amount of water on the earth and from the absence of processes 

 which could alter, and especially decrease, the total amount of water present. A 

 stationary water cycle requires that the amount of water passing through any par- 

 ticular part of this cycle (as either liquid, solid or vapour) should not vary with time, 

 and particularly that the amount of water entering the cycle by evaporation from the 

 ocean is returned to it. In that way there is never any permanent gain or loss of water 

 from any point of the cycle. 



For a quantitative assessment of the water cycle on the Earth it is necessary to make 

 a numerical estimate of the amount of water circulating through it. This can be done 

 either at the place where water reaches the surface of the Earth from the atmosphere 

 (precipitation), or where water leaves the Earth's surface in form of water vapour 

 (evaporation). In both cases the numerical basis necessary for an estimate must be 

 obtained from observations. On the continents the amount of water vapour precipi- 

 tated from the atmosphere can be determined with sufficient accuracy by direct mea- 

 surement of the precipitation, and this quantity can be determined more accurately 

 the denser the network of rainfall measuring stations. The determination of the mean 

 precipitation amount over the sea is, on the other hand, very difficult and is never 

 precise because of uncertainties in the measurement of precipitation on boardship. 

 On the other hand, the accurate determination of the amount of mean evapora- 

 tion on the continents is accompanied by considerable difficulty while the direct 

 determination of the evaporation from the oceans seems possible and can be made 

 more easily because of the more uniform conditions at this surface, however, in 

 practice critical examination is still needed. These circumstances give particular 

 importance to the question of the magnitude of evaporation amount from different 

 regions of the oceans. 



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