60 THE HEAT BUDGET OF THE OCEANS 



the time of the day and of the season of the year, in contrast to the 

 incoming short-wave radiation from the sun and the sky, which is sub- 

 jected to very large diurnal and seasonal variations. In the presence of 

 clouds the effective back radiation is cut down, because the radiation from 

 the atmosphere is increased. The empirical relation can be written 



Q = Qo(l - 0.083C), (IV, 5) 



where Qo is the back radiation at a clear sky and where C is the cloudiness 

 on the scale 1 to 10. A diurnal or annual variation in the cloudiness will 

 lead to a corresponding variation in the effective back radiation. On an 

 average, the diurnal variation of cloudiness over the oceans is very 

 small and can be neglected, but the annual variation is in some regions 

 considerable. The above equation is applicable to average conditions 

 only, because the reduction of the effective back radiation due to clouds 

 depends upon the altitude and density of the clouds. Thus, if the sky is 

 completely covered by cirrus, alto stratus, or strato cumulus clouds, the 

 effective radiation is about 0.75 Qo, 0.4 Qo, and 0.1 Qo, respectively. 



The Radiation Budget of the Oceans. The annual incoming 

 short-wave radiation from the sun and the sky is greater in all latitudes 

 than the outgoing effective back radiation. According to Mosby the 

 average annual surplus of incoming radiation between latitudes 0° and 

 10° N is about 0.170 g cal/cmVmin, and between 60° and 70° N, about 

 0.040 g cal/cm^/min. The surplus of radiation must be given off to the 

 atmosphere, and the exchange of heat and water vapor with the atmos- 

 phere is therefore equally important with the processes of radiation in 

 regulating the ocean temperature and salinity. 



The characteristics of the oceans in respect to radiation are very 

 favorable to man. The water surface reflects only a small fraction of 

 the incoming radiation and the greater part of the radiation energy is 

 absorbed in the water, distributed by processes of mixing over a layer of 

 considerable thickness, and given off to the atmosphere during periods 

 when the air is colder than the sea surface. The oceans therefore exercise 

 a thermostatic control on climate. Conditions are completely changed, 

 however, if the temperature of the sea surface decreases to the freezing 

 point, so that further loss of heat from the sea leads to formation of ice, 

 because when passing this critical temperature the characteristics are 

 altered in a very unfavorable direction. Sea ice, which soon attains a 

 gray-white appearance owing to enclosed air bubbles, reflects 50 per cent 

 or more of the incoming radiation, and if covered by rime or snow the 

 reflection loss increases to 65 per cent, or even to 80 per cent if covered by 

 fresh, dry snow. The snow surface, on the other hand, radiates nearly 

 like a black body, and consequently the heat budget related to processes 

 of radiation, instead of rendering a surplus, as it does over the open 

 ocean, shows a deficit until the temperature of the ice surface has been 



