WATER 101 



by the mere surface alone, but a considerable 

 layer of water near the surface receives the 

 heat. 



At the equator the evaporation of the ocean 

 appears to be about 2.3 meters per year, 1 

 which involves more than 1,000,000,000,000,- 

 000 calories of latent heat per square kilo- 

 meter. The amount of heat which is em- 

 ployed in evaporating water from 100 square 

 kilometers of the tropical ocean is accordingly 

 vastly more than all the energy employed in 

 the metabolism of the total population of 

 the United States, and it amounts to more 

 than 100,000,000 horse power. This is equiv- 

 alent to more than one horse power per square 

 meter day and night throughout the year. 

 To a greater or less extent all over the earth 

 this same process goes on, and as a result the 

 water vapor in the air probably averages 

 between 15 and 20 kilograms per square meter 

 of the earth's surface, an ample supply for 

 the formation of rain. The effect of this 

 enormous evaporation to moderate the tem- 

 perature of the tropics is very considerable; 

 but the heat which thus disappears is not 

 lost. Rendered latent at the place of evap- 

 oration, it is turned back into actual heat at 



1 This and other similar facts will also be found in the 

 work of Hann. 



