THE HEAT BUDGET OF THE OCEANS 61 



lowered so much that the decreased loss by effective back radiation 

 balances the small fraction of the incoming radiation that is absorbed. 

 The immediate result of freezing is therefore a general lowering of the sur- 

 face temperature of the ice and a rapid increase of the thickness of the ice. 

 The air that comes in contact with the ice is cooled, and, as this cold air 

 spreads, more ice is formed. Thus, a small lowering of the temperature 

 of the water in high latitudes followed by freezing may lead to a rapid 

 drop of the air temperature and a rapid increase of the ice-covered area. 

 On the other hand, a small increase of the temperature of air flowing in 

 over an ice-covered sea may lead to melting of the ice at the outskirts 

 and, once started, the melting may progress rapidly. In agreement with 

 this reasoning, it has been found that the extent of ice-covered areas 

 in the Barents Sea is a sensitive indicator of small changes in the atmos- 

 pheric circulation and of small changes in the amount of warm water 

 carried into the region by currents. It has also been computed that if 

 the average air temperatures in middle and higher latitudes were raised a 

 few degrees, the Polar Sea would soon become an ice-free ocean. 



Exchange of Heat between the Atmosphere and the Sea 

 The amount of heat that in unit time is carried away from the sea 



surface through a unit area is equal to —Cp^e l-r- — 7 ), where Cp is the 



specific heat of the air, ^e is the eddy conductivity, — di^/dz is the temper- 

 ature gradient of the air (the lapse rate), which is positive when the 

 temperature decreases with height, and 7 is the adiabatic lapse rate. 

 Very near the sea surface, 7 can be neglected as small compared to d^/dz. 

 The term Cp/Xe enters instead of the coefficient of heat conductivity of the 

 air as determined in the laboratory, because the air is nearly always in 

 turbulent motion and because in the air fis = y^e (p. 19). The state of 

 turbulence varies, however, with the distance from the sea surface, 

 because at the surface itself the eddy motion must be greatly reduced. As 

 a consequence, under steady conditions, when the same amount of heat 

 passes upward through every cross section of a vertical column, the 

 temperature changes rapidly with height near the sea surface and 

 more slowly at a greater distance. The product —Cpiied^/dz remains 

 constant and, since CpHe increases rapidly with height, —dd/dz must 

 decrease. 



Detailed and accurate temperature measurements in the lowest 

 meters of the air over the ocean have not yet been made, because the hull 

 and masts of a vessel disturb the normal distribution of temperature to 

 such an extent that values observed at different levels on board a vessel 

 are not representative of the undisturbed conditions. The few measure- 

 ments that have been attempted indicate, however, that the general 

 distribution as outlined above is encountered. 



