pile ye 
to the atmosphere and to space, less the radiation from the atmos- 
phere to the.surfaee. It is operative at all times, but its name 
comes from the fact that at night, when solar radiation is absent, 
it accounts for the total radiation transfer at the surface. This 
is long-wave radiation, for which water is very opaque. On the 
other hand a large part of the energy of solar radiation is in the 
visible wave lengths, for which water is relatively transparent. 
Of the solar radiation absorbed by the sea at least 5% is absorbed 
in the first tenth of a millimeter, 14% in the first millimeter, 
27% in the first centimeter and 45% in the first 10 centimeters; in 
the first meter 64% to 90% is absorbed.* 
Solar radiation adds heat to the surface water. Evaporation 
(except when it is negatives condensation on the sea surface) takes 
heat from it, as does usually nocturnal radiation. Conduction be- 
tween the air and the water may be in either direction. The net 
result of these factors determines whether the water is gaining or 
losing heat. 
The effect on the vertical temperature gradient of heating or 
cooling the surface water depends on the water's stability, or in- 
erease per unit length downward of density. Stability can have a 
wide range of vositive values, but it can be only slightly negative 
(except in a very thin layer). If there is negative stability, or 
instability, in a layer, convection occurs, mixing the water in the 
layer and maintaining the instability at a low value. Thus there 
are essentially two types of layer, the one of nearly zero stability, 
or of nearly neutral equilibrium, and the stable layer. Ina stable 
a en a ar re a ee a NT NS NE YAO ON A I TAS ATA TTS RT I ee 
* Dorsey, Properties of ordinary water-substance, ppe 333-334. 
