a eee 
The absorption increases as the altitude of the sun decreases, 
With the sun in the zenith the solar beam vasses throuzh one "atmos- 
phere", with the sun at an altitude of 30° it passes through 2 "at- 
mospherés",at 19° through three, at 14° through four, etc. Thus 
the proportion of solar radiation absorbed in the atmosphere varies 
throughout the day; for ecually transparent air the proportion ab- 
sorbed is least at noon and becomes very sreat when the sun is near 
the horizone 
The depletion of solar radiation in the atmosphere is due mostly 
to the presence of water vapor and dust. With dust-free air and with 
the sun in the zenith the vroportion of solar radiation lost in 
transmission to the sea surface varies between about 20 and 35 per 
cent. For various altitudes of the sun and for two specified amounts 
of water vaoor in the atmosphere the amounts of solar radiation re- 
ceived on a horizontal surface are given in Table 4. 
For present purvoses the cuantity usually desired is the total 
solar radiation received at the sea surface on a given day at a given 
spot. Since the instantaneous values of Table 4 are laborious to 
integrate throughout the day,it is more convenient to use average 
daily totals for various latitudes and seasons as presented in 
Table 5, 
The effect of clouds is to reduce the daily total solar radiation 
received at the sea surface, althoush a momentary increase may be 
effected when the sun is unobscured but surrounded by clouds. With 
an overcast sky the amount of reduction depends on the density of 
the clouds; it can be small with a thin cirrostratus cover, and with 
cumulonimbus the radiation can be reduced to one ver cent. A useful 
expression for the ratio of the solar radiation received at the sea 
surface under average conditions to the radiation with cloudless 
