THE HEAT BUDGET OF THE OCEANS 



59 



Angstrom has published a table that summarizes the results of ob- 

 servations of effective radiation against a clear sky from a black body 

 of different temperatures and at different vapor pressures. Fig. 11 has 

 been prepared by means of this table, taking into account the small 

 differences between the radiation of a black body and that of a water 

 surface. The figure shows the effective radiation as a function of sea- 

 surface temperature and of relative humidities between 100 per cent and 

 70 per cent, but the values that can be read off from the graph may be 

 10 per cent in error, owing to the scanty information upon which the 

 curves are based. It brings out the fact, however, that, owing to the 

 increased radiation from the atmosphere at higher temperatures (higher 

 vapor pressure), the effective back radiation decreases slowly with increas- 



Fig. 11. The effective back radiation in gram calories 

 per square centimeter per minute from the sea surface to 

 a clear sky, represented as a function of sea surface tem- 

 perature and relative humidity of the air at a height of 

 a few meters. 



ing temperature. At a temperature of 0°C and a relative humidity of 

 80 per cent, the effective back radiation decreases with increasing 

 humidity, owing to the increasing back radiation from the atmosphere. 

 Thus, at a surface temperature of 15° the effective radiation is about 

 0.180 g cal/cm^/min at a relative humidity of 70 per cent, and about 

 0.163 g cal/cm^/min at a relative humidity of 100 per cent. 



The values of the effective back radiation at higher temperatures as 

 obtained by extrapolation of Angstrom's data (fig. 11) are greater than 

 those computed from Brunt's empirical formula 



Q, = Q'(l - 0.44 - 0.08 Vi), 



where Q' is the radiation of a black body having the temperature of the 

 sea surface and e is the vapor pressure of the air in millibars. However, 

 in this formula the numerical values of the coefficients are uncertain and 

 are applicable only within a range of e between 4 and 18 millibars. 



The diurnal and annual variations of the sea-surface temperatures and 

 of the relative humidity of the air over the oceans are small, and the 

 effective back radiation at a clear sky is therefore nearly independent of 



