230 Evaporation from the Surface of the Sea and the Water Budget of the Earth 



cm 

 0-30 



0i20 



0-0 



Yvr 0-01 0-02 0-03 004 0-05 0-06 0-07 0-08 





Fig. 103a. Relationship between the thickness of the diffusion layer (d) and the reciprocal 

 of the shearing-stress velocity (1/w*). 



correction method to the direct measurements of the evaporation on board ships. 

 Taking, according to the preceding section, the mean annual evaporation height 

 for the Atlantic as 100 cm then the values given by Wust,and shown in Table 89 having 

 a mean evaporation of 83 cm, must be multiplied by the factor 1-22 (they have to be 

 divided by 293 if values in mm/day are needed). 



Table 89. Vahies of evaporation for zonal regions of the Atlantic found by calculation 

 from the meteorological data and from observations of evaporation 



t Taking into account the factor 0-98 as the effect of salinity 



Table 89 presents this calculation, and a comparison between calculated and 

 observed £■ values. Figure 104 shows the results graphically. The agreement between tne 

 observed and the calculated values is rather good, and in any case considerably better 

 than in the second case treated by Sverdrup for a smooth surface without any diffusion 

 layer. This agreement shows that the theory of a diffusion-layer with a thickness de- 

 creasing with increasing wind velocity and with a turbulent layer above it with a 

 roughness parameter Zq = 0-6 cm is capable of explaining the evaporation at the sur- 

 face of the sea. However, all the assumptions so far are based on very few observa- 

 tions, so that further support by systematic measurements would be extremely de- 

 sirable. The evaporation formula on p. 228 shows in any case that the dependence of 

 the evaporation on the meteorological conditions in the atmosphere above it is more 

 complicated than was assumed in previous relationships, and that deeper insight into 

 these phenomena can only be obtained by a geophysical analysis of the evaporation 

 process. 



