OCEANOGRAPHY 431 



100 metres. This is a typical winter condition, and is due 

 to the vertical circulation already mentioned, which is caused by 

 the surface water being cooled in winter, thus becoming heavier 

 than the water below, so that it must sink and give place to lighter 

 water which rises. In this way the upper zones of water become 

 mixed, and acquire almost equal temperatures and salinities. It 

 thus appears that the vertical currents reached a depth of about 

 100 metres in July, 1911, in the central part of the South Atlantic. 

 This cooling of the water is a gain to the air, and what happens 

 is that not only the surface gives off warmth to the air, but also the 

 sub-surface waters, to as great a depth as is reached by the vertical 

 circulation. This makes it a question of enormous values. 



This state of things is clearly apparent in the sections, where 

 the isotherms and isohalins run vertically for some way below the 

 surface. It is also clearly seen when we draw the curves of dis- 

 tribution of salinity and temperature at the different stations, as 

 we have done in the two diagrams for Stations 32 and 60 (Fig. 9). 

 The temperatures had fallen several degrees at the surface at the 

 time the FrarrCs investigations were made. And if we are to 

 judge from the general appearance of the station curves, and from 

 the form they usually assume in summer in these regions, we shall 

 arrive at the conclusion that the whole volume of water from the 

 surface down to a depth of 100 metres must be cooled on an 

 average about 2° C. 



As already pointed out, a simple calculation gives the following: 

 if a cubic metre of water is cooled 1 C, and the whole quantity 

 of warmth thus taken from the water is given to the air, it will 

 be sufl&cient to warm more than 3,000 cubic metres of air 1° C. 

 A few figures will give an impression of what this means. The 

 region lying between lats. 15° and 35° S. and between South 

 America and Africa — roughly speaking, the region investigated 

 by the Fram Expedition — has an area of 13,000,000 square kilo- 

 metres. We may now assume that this part of the ocean gave off 

 so much warmth to the air that a zone of water 100 metres in 



