136 Temperature 



The data show that the atmosphere absorbs radiation of short wave- 

 length faster than that of long wavelength, whereas water is most 

 transparent to radiation in the central part of the visible region. In 

 all parts of the spectrum radiation is absorbed by water at far higher 

 rates than by air, and the contrast is particularly great in the 

 infrared. The absorption by a few meters of water is enormously 

 greater than by several thousand meters of air. In general we can 

 say that in comparison with the atmosphere the water medium is al- 

 most opaque to the sun's radiation. 



Because of the differences in mobility and transparency of the 

 two media we find that the vertical temperature changes in the water 

 environment are controlled differently from those in the air. Radia- 

 tion from the sun is the only important source of heat for natural 

 bodies of water. Tests have shown that no significant exchange of 

 heat takes place between the water and the mud of the bottom. Since 

 solar radiation is absorbed in the upper strata of water, water bodies 

 receive heat only at their upper surfaces. Cooling of water can take 

 place through radiation, evaporation, or the melting of ice. All of 

 these processes occur at the surface in natural water bodies. Gen- 

 erally speaking, the seas and the inland waters gain and lose their 

 heat primarily at the top. 



It is not surprising to find as a consequence of the foregoing that in 

 deep bodies of water the major changes in temperature are limited 

 to the surface strata. In some situations these changes are superim- 

 posed on a deeper, permanent temperature structure. In the tem- 

 perate and tropical regions of the sea a permanent thermal gradient 

 has been produced between the mixed layer near the surface and the 

 deep layer filling the bottom of ocean basins. The depth and extent 

 of the zone of relatively rapid temperature change, known as the 

 permanent thermocline layer, differ in the various parts of the ocean 

 but an illustrative example is given in Fig. 5.4 for water in the great 

 central eddy of the Atlantic Ocean. From roughly 2000 m down 

 to the bottom the water is everywhere just about 3°C. Since the 

 average depth of the ocean is approximately 4000 m, there is ob- 

 viously a very large amount of space in which the temperature is uni- 

 form month in and month out, year in and year out. If you were a 

 fish restricted to 3°C, you could nonetheless travel over more than 

 60 per cent of the globe without being exposed to a significantly dif- 

 ferent temperature. 



Although essentially all the incident radiation is received at the 

 surface of natural water bodies, we know that some heat is eventually 

 transferred to deeper layers. How does the heat get there? Radia- 



