The Hydrosphere 



171 



to bottom. In winter the water is coldest on the surface 

 and in the river, growing warmer, at first rapidly, and then 

 more gradually, toward the sea. In spring, on account of 

 the land heating up more rapidly, the temperature becomes 

 once more uniform throughout. 4 Fig. 32 shows the actual 



FIG. 32. Temperature of surface water at different seasons along the middle line 

 of the Firth of Forth. Distances from Alloa are shown in miles horizontally 

 from left to right ; temperature in degrees Fahrenheit is shown vertically. 



distribution of temperature along the Firth of Forth, from 

 Alloa to the sea, at four typical seasons, on the surface. 



233. Surface Temperature of the Ocean. The iso- 

 thermal lines on the ocean in Plate XV. represent the 

 average temperature of the surface water for the year. Al- 

 though more easily heated than fresh water ( 227), the sea 

 surface has a less range of temperature than that of fresh 

 lakes. This results in part from the greater clearness of 

 sea-water, in part from its distance from heated land. The 

 average temperature of the surface of the open ocean varies 

 less than i between day and night, but between summer 

 and winter there is a range of from 5 to 10. Along a 

 line, drawn from Newfoundland to Iceland, the annual' 

 change of temperature between the colclest rnonth, February, 

 and the hottest month, August, is as much as 20; but this 

 is due less to the heating and cooling of water than to a 

 seasonal change in direction of warm and cold currents 

 ( 242). In the tropical zone the sea surface has a temperature 

 higher than 80 for the whole year. This zone of very hot 

 water is widest in the Indian Ocean and narrowest in the 

 Atlantic ; and in all three oceans it is wider on the western 

 than on the eastern shores. The temperature falls very 

 uniformly toward the south, reaching 40 F. about latitude 

 48 S. south of Africa, but not until latitude 58 S. south of 



