70 THE HEAT BUDGET OF THE OCEANS 



night and minima at sunrise and in the early afternoon hours. It is 

 possible that the afternoon minimum appears exaggerated, owing to 

 uncertainties as to the absolute values of g^ and q^. The total diurnal 

 evaporation was 0.5 cm, but the sky was nearly clear on the four days 

 that were examined, and the average diurnal value is therefore smaller. 

 The double diurnal period of evaporation appears to be characteristic 

 of the Tropics, but in middle latitudes a single period with maximum 

 values during the night probably dominates. 



It may be added here that the annual variation of evaporation from 

 small inland lakes is quite different from that from the oceans, in general 

 reaching maximum values in summer. There are many reasons for this 

 difference. In the first place, the annual range of the lake-surface 

 temperature is great, the temperature varying between, say, 25° and 0°. 

 In the second place, the air blowing over a lake is often dry, and its 

 humidity content will not increase materially when passing over a lake 

 of small or moderate size. As a consequence, the difference Cw — Ca will 

 be great in summer and small in winter, although in summer the air may 

 be warmer than the lake surface and in wdnter it may be colder. As 

 examples, assume (1) that in summer the lake surface temperature is 

 20°, the air temperature is 25°, and the relative humidity is 40 per cent, 

 and (2) that in winter the lake surface temperature is 5°, the air tempera- 

 , ture is 0°, and the relative humidity is again 40 per cent. In the former 

 case Bw — Ca equals 10.7 millibars, whereas in the latter case Cw — ea 

 equals 7.6 millibars, and at the same wind velocity evaporation is greater 

 in summer. If the lake freezes, evaporation is greatly reduced. 



