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



of topography above the values for the open ocean. This probably is not too far from 

 the actual conditions. 



For the entire Earth, according to recent calculations (Reichel, 1952), the 

 mean annual precipitation is about 86 cm/year, which under stationary conditions is 

 balanced by an equally large evaporation amount. Therefore the average evaporation 

 amount for the whole Earth amounts to 2-37 mm water per day. An interesting graphi- 

 cal representatation of the total hydrologic cycle has been given by Lettau (1954), 

 and is shown in Fig. 105. It gives detailed information on all aspects of this cycle. 



ATMOSPHERE 



L I THOSPHERE 



Fig. 105. Schematic diagram of the hydrologic cycle. 100 relative units = 85-7 g cm~* year" 

 or 857 mm global annual mean of precipitation (according to Lettau, 1954). 

 1 ^ , Evaporation; 2 • • •■► , Precipitation; 3 _ — — > , Dew deposit; 



of water vapour; 6 e = values smaller than 0-5 rel. units. 



4 o o o o ot> , Run off; 



Removal from and addition to horizontal advection 



6. Energy Budget between Ocean and Atmosphere for DiflFerent Oceans and Oceanic 

 Regions 



The heat turnover between the total ocean and the total atmosphere has already been 

 discussed in previous chapters. It is also of considerable interest to know the energy 

 budget between the ocean and the atmosphere for the individual oceans and for differ- 

 ent parts of the ocean, since on this depend the effects of the sea on the atmosphere 

 above it or, in turn, the influences of the atmosphere on the sea. Such investigations, 

 in spite of their importance, have only recently been made and indeed have been car- 

 ried out almost exclusively by Jacobs (1942, 1943, 1951^, h) and Albrecht (1949, 

 1951). These investigations are based on the calculation of the evaporation from the 

 formula on p. 230 using the differences ^^ — 'da and e^ — e^ derived from climatologi- 

 cal charts of the oceans. Doubts about these latter values have been expressed by 

 Dietrich (1950), but it appears that any errors that may have been introduced in this 

 way are not systematic but may vary from one region of the sea to another and 

 should, at least in part, cancel out. Calculations of this type have been made especially 

 for the North Atlantic and the North Pacific, for which the climatic charts are more 

 reliable. Such calculations of course give only a rough estimate but they serve, however, 

 to give an approximately quantitative idea of the interplay between ocean and atmos- 

 phere. At first the most important is the pure heat gain by the radiation turnover 

 Qs — Qb, whereby Qg is the absorption of solar and sky radiation and Qi, is the radia- 

 tion loss from the sea surface. 



Figure 106 shows the geographical distribution according to Sverdrup (1943) of the 

 annual surplus of radiation penetrating the water surface. Over the whole year the 

 oceans have everywhere a gain of heat from radiation, but north of 25° N. this gain 

 decreases rapidly with latitude, therefore from 10° to 45° N. it is smaller on the eastern 



