THE HEAT BUDGET OF THE OCEANS 51 



Radiation 



Incoming Radiation; Effect of Clouds; Reflection. Part of the 

 short-wave radiation that reaches the sea surface comes directly from the 

 sun and part of it comes from the sky as reflected or scattered radiation. 

 The amount of radiation energy that is absorbed per unit volume in the 

 sea depends upon the amount of energy which reaches the sea surface, 

 the reflection from the sea surface, and the extinction coefficients for 

 total energy. The incoming radiation depends mainly upon the altitude 

 of the sun, the absorption in the atmosphere, and the cloudiness. With 

 a clear sky and a high sun, about 85 per cent of the radiation comes 

 directly from the sun and about 15 per cent from the sky, but with a 

 low sun the proportion from the sky is greater, reaching about 40 per cent 

 of the total with the sun 10 degrees above the horizon. 



The incoming energy from the sun is cut down in passing through 

 the atmosphere, partly owing to absorption by water vapor and carbon 

 dioxide in the air, but mainly by scattering against the air molecules or 

 very fine dust. The total effect of absorption and scattering in the 

 atmosphere depends upon the thickness of the air mass through which 

 the sun's rays pass, as expressed by the equation 



J = g^-Bamm^ (IV, 3) 



Here / represents the energy in g cal/cmVmin reaching a surface that is 

 normal to the sun's rays; m represents the relative thickness of the air 

 mass and is equal to 1 at a pressure of 760 mm when the sun stands in 

 zenith, equal to 2 when the sun is 30° above the horizon (sin 30° = J^), 

 and so on; >S is the solar constant and is equal to 1.94 g cal/cmVmin; B 

 is the ''turbidity factor" of the air; and a^ = 0.128 — 0.054 log m. 



The sun's radiation on a horizontal surface is obtained by multiplica- 

 tion with sin h, where h is the sun's altitude. To this amount must be 

 added the diffuse sky radiation in order to obtain the total radiation on 

 a horizontal surface. Instruments are in use for recording the total 

 radiation and for recording separately the radiation from the sun and 

 the sky. 



When the sun is obscured by clouds, the radiation comes from the 

 sky and the clouds, and on an average can be represented by the formula 

 Q = Qo(i — 0.071C), where the cloudiness C is given on the scale to 10, 

 and where Qo represents the total incoming radiation with a clear sky. 

 This formula is applicable, however, only to average conditions. If the 

 sun shines through scattered clouds, the radiation may be greater than 

 with a clear sky, owing to the reflection from the clouds, and on a com- 

 pletely overcast, dark and rainy day the incoming radiation may be cut 

 down to less than 10 per cent of that on a clear day. Table 6 contains 

 the average monthly amounts of incoming radiation, expressed in gram 



