4 SEA-BIRDS 



nesting between as temperate. However, the temperature of neither air 

 nor water arranges itself, in the Atlantic, according to latitude.* For 

 instance, if we examine the July air isotherms over the world north 

 of the Tropic of Cancer we see that that for 45° F. runs well south of the 

 Arctic Circle in the areas Greenland-Baffin Island and Bering Strait, 

 and well north of it off Scandinavia, avoiding Lapland altogether. 



If we examine a map of the world (showing particularly the 

 lands between the Tropics), we see that the summer isotherm for 

 80 °F. (July in the northern hemisphere, January in the southern) 

 runs well north of the Tropic of Cancer in Mexico and the southern 

 States, and in Africa and Asia, and south of the Tropic of Capricorn 

 in Africa and Australia; yet large parts of the tropical Pacific and 

 Atlantic Oceans never reach an average summer air temperature of 8o°F. 



In the North Atlantic there is not only relatively little direct 

 correspondence between isotherms and latitude, but there is a 

 good deal of difference in position between the same isotherms under 

 the surface, on the water surface and in the air. 



The primary cause of the ocean currents, and of the prevailing 

 winds which are associated with them, is the rotation of the earth. 

 The plot of the Atlantic currents and Atlantic winds is almost, though 

 not quite, coincident. To a very large extent the distribution of 

 Atlantic water temperatures, and to a large extent that of air 

 temperatures, is a consequence of these currents and prevailing winds. 

 However, in parts of the Atlantic evaporation and the melting of 

 ice produces temperature and salinity gradients which themselves 

 produce consequent currents. Hence the web of sequence and con- 

 sequence, of cause and effect, becomes complex. We must examine 

 the great equatorial current first, for almost every one of the more 

 important sea masses in the Atlantic owes its existence to it. It is 

 quite justifiable to write in terms of sea masses, for, as we shall see, 

 the Atlantic waters are by no means homogeneous and can be divided, 

 sometimes with strikingly sharp boundaries, into volumes possessing 

 very diverse properties. 



We need scarcely remind the reader that if he faces a globe, poised 

 in the ordinary way with North at the top, and spins it as the earth 

 naturally rotates, the points on its surface will travel, as they face him, 



*Maps of the northern part of the North Atlantic showing July surface-water 

 isotherms, air-isotherms, vapour-pressure and relative humidity, and annual rainfall, 

 are given by Fisher (1952, pp. 284-85); and a good map of August surface-water 

 isotherms by Storer (1952, p. 186). 



