242 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 70 



EVAPORATION AND TEMPERATURE 



According to our thought, erroneous views have also often been 

 entertained on precipitation and evaporation. Authors have as- 

 sumed that increased radiation and consequently increased tempera- 

 ture in the atmosphere must always correspond to increased evapora- 

 tion and therefore increased precipitation. This is, however, not 

 the case. Increased precipitation must, to be sure, on the whole ac- 

 company increased evaporation of the surface of the ocean or the sur- 

 face of the land. Increased evaporation again one might think must 

 be accompanied by increased temperature, but this is not always 

 the case. Evaporation of the surface, whether of the ocean or of 

 the land, is obviously dependent not only on the temperature, but 

 also on the vertical and horizontal circulation of the atmosphere. 

 Suppose there is little movement prevailing in the atmosphere, and 

 its temperature is besides relatively high, and higher or at least not 

 appreciably lower, than the temperature of the surface of the 

 ocean. Under these circumstances, even if the temperature of 

 both air and ocean were relatively high, there would be compara- 

 tively small evaporation since the air layers nearest the ocean sur- 

 face would be very quickly saturated. Not being warmer than the 

 air layers immediately above them they would not rise, but would lie 

 upon the ocean and hinder further evaporation. On this account 

 the evaporation can be relatively small at very high temperatures. 

 This is exactly the condition which often occurs in summer when 

 the air temperature is as high or even higher than the surface 

 temperature. 



If on the other hand the ocean surface is considerably warmer 

 than the air, then even if there was not very great circulation in the 

 atmosphere itself, vertical convection would arise in these conditions. 

 The lowest air layer would be warmed and rise to greater heights 

 and would be displaced by new layers which would in turn be loaded 

 with moisture, and the evaporation would go on relatively fast even 

 if no other motion of the air prevailed. That is the condition which 

 occurs during the colder parts of the year very generally. It also 

 happens that at these times a strong horizontal motion of the atmos- 

 phere prevails, so that one must assume that the evaporation at such 

 times is quite considerable, and probably greater than that in the 

 average of the warmest part of the year. Certainly the precipita- 

 tion in winter is on the whole greater than that of summer. Also 

 on this point Newcomb goes from not entirely correct assumptions 

 when (1908, p. 394) he assumes that fluctuations of temperature on 



