PHYSICAL OCEANOGRAPHY 229 



at this depth the seasons were reversed : it was " winter " in 

 the water in the middle of the summer, and "summer" in the 

 middle of the winter. Murray's observations in Upper Loch 

 Fyne in 1888 gave similar results. At 300 metres at the 

 " Michael Sars " Station there were hardly any variations at all, 

 the temperature being very much the same as the mean annual 

 temperature of the air, as Nordgaard has shown to be the case 

 with regard to the bottom-water of the Norwegian tjords. 



When sea-water is cooled its density increases, and it often vertical 

 happens in winter that the surface-water becomes heavier than "^'g^^^'wat'erl 

 the water below. The surface-layer then sinks, and the under- 

 lying water comes to the surface. By this vertical circulation 

 the conditions are equalised, so that exactly the same salinities 

 and temperatures are found as far down as the vertical circulation 

 extends ; wind and current aid in the process. This takes place 

 especially from January to March ; in April the weather again 

 becomes warmer and the temperature begins to rise at the 

 surface. A very good example of this phenomenon is afforded 

 by the "Michael Sars" observations taken to the westward of 

 Plymouth in April 1910 ; at the very surface the temperature 

 had risen slightly, but otherwise practically the same salinities 

 and temperatures prevailed at every station down to a depth of 

 150 metres or more. Later on in spring the surface tempera- 

 ture gradually rises, and a marked discontinuity-layer is formed. 

 In many places near the coast, where the salinity is low at the 

 surface and high beneath the surface, a brisk vertical circulation 

 cannot be set up ; the comparatively fresh water on top is so 

 light that, even when considerably cooled, it does not change 

 places with the salt and heavy water below. But farther out 

 to sea the vertical circulation may extend down to a depth of 

 200-300 metres or more. 



It is thus not only the surface-water that may give off heat Effect of heat 

 to the air, but a great body of water extending to several f^eTea.^^^ 

 hundred metres in depth, and hence the great influence of the 

 sea on winter climates. The capacity for heat of water is very 

 great compared with that of the air. Supposing that we have 

 I cubic metre of water giving off enough heat to the air to 

 lower the temperature of the water one degree, this heat would 

 be sufficient to raise the temperature of more than 3000 cubic 

 metres of air by one degree. An example will show the 

 importance of this. Suppose a body of water, 700,000 square 

 kilometres in extent and 200 metres deep, to give off enough 

 heat to the air in winter to lower the water-temperature one 



