74 A TEXTBOOK OF OCEANOGRAPHY 



although the early estimates of the difference were much too 

 high. 



The best comparison between the evaporation of salt and 

 fresh water is probably that made by the Japanese investigator, 

 Okada, whose researches extended over seven years. He found 

 that, on the average, sea-water evaporation was about 95 per 

 cent, of that of fresh water, the annual variations ranging from 

 92-6 to 96*8 per cent. 



In Japan the average daily evaporation of sea-water was 

 3-44, and for fresh water 3*27 mm., the maximum being in 

 August (6'00 and 5-69 mm.), the minimum in January (1-97 

 and ro3 mm.). 



Okada also -investigated carefully the influence of the air 

 temperature and sunlight on evaporation. 



Textbooks on meteorology give the following chief 

 influences on the rate of evaporation : 



1. Atmospheric pressure. 



2. Atmospheric temperature. 



3. Atmospheric humidity. 



4. Atmospheric movements e.g., wind. 



5. Variations in water salinity. 



Evaporation takes place more quickly at a lower pressure 

 of the air, and also the higher the temperature. The less the 

 humidity the greater the evaporation. With strong winds the 

 evaporation also increases, and, as already stated above, the 

 greater the salinity the less the evaporation. Sunlight has 

 also an important effect on promoting evaporation, and 

 according to Okada evaporation proceeds on the average 

 nearly two and a half times more quickly in sunlight than in 

 the shade. 



This agrees with Mill's results, obtained on observation of 

 the evaporation of fresh water in Camden Square, London, 

 where the evaporation was found to be directly dependent on 

 the amount of sunlight. 



