Pl Cy 
decreases linearly with distance aiong the beam, the proportionality 
factor being the absorption coefficient. 
The absorption coefficient of water (or air) varies widely over 
the spectrum of solar radiation. Since some wave:-lengths : Spar 
are entirely absorbed in the first centimeter while others are ab- 
sorbed only slightly, the simple absorption law does not hold for 
the total intensity. When the absorption coefficient varies over the 
spectrum of radiation vresent, each successive layer of equal thick- 
ness absorbs a successively lesser fraction of the radiation imping- 
ene Upon Lt. 
Another concept of importance is Kirchhoff's law. One way of 
stating this is that for a given wave length, if a body absorbs a 
eertain fraction of the incident radiation, it emits the same fraction 
of the radiation an ideal radiator at its temperature would emit. Of 
the radiation emitted by the ocean surface -- the continuous spec- 
trum of an ideal radiator -- certain bands are strongly absorbed by - 
the atmosphere, for certain other bands a cloudless atmsophere is 
transparent. From Kirchhoff's law it is apparent that the atmos»nhere 
radiates back to the ocean in the bands where if absorbs, but gives 
off no radiation in the bands where it is transparent. The atmosphere 
is never enough warmer than the water to radiate in its limited spec- 
trum so much energy as is in the complete spectrum of the ocean sur- 
face. Hence in cloudless weather the nocturnal radiation over water 
is always positive: the water radiates more energy than it receives 
from the atmosphere. 
Clouds emit the continuous spectrum of an ideal radiator. If 
the sky is completely covered with stratus clouds at the same temper- 
ature as the sea surface, the clouds send down just as much radiation 
as the water sends up. Since the clouds are usually colder than the 
