SOLAR RADIANT ENERGY 21 
ereased linearly with p”. For the minimum points 
(1.87 » and 1.90 1) the same pressure relation existed up 
to p = 400 mm Hq; for higher pressure the absorption 
increased more slowly as p” increased. For each of 
the entire bands at 1.35 uw and 1.85 », Chapman, Howard, 
and Miller [19] find that the fractional transmission in- 
creases as a nonlinear function of (p + pw)! where 
Pw is the partial pressure of the water vapor. They do 
not graph their functional absorption relation explicitly 
as a function of pressure. 
TasBiLE IJ. Per Cent ABSORPTION OF RADIATION BY WATER 
VAPOR 
(After Fowle [27]) 
Wwoveclenethinterval Precipitable water vapor 
(H) 0.008 cm 0.082 cm 
To % 
1.3 -1.75 6.1 18 
1.75-2.2 13.6 29 
2.2 -3.2 23.6 41 
3.2 -4.0 PANT 37 
4.0 -4.9 32.5 50 
4.9 -5.4 18 42 
5.4 -5.9 47 85 
5.9 -6.4 64 97 
6.4 -7.0 68 97 
7.0 -8.0 25 62 
The amount of solar energy beyond 3 wu is of course 
very small, and hence the absorption of that energy 
cannot greatly affect the temperature of the atmo- 
sphere. Karandikar [50] shows the amount of absorp- 
tion in langleys per second in the various wave-length 
bands as a function of precipitable water vapor w for 
values of w from 10~4 cm to 1 em and for bands in the 
region from 0.9 » to 8 yu; he also gives the total amount 
of energy absorbed by water vapor in the stratosphere 
and shows that above 40 km the absorption is practic- 
ally zero, but that at 30 km it approaches absorption by 
ozone (especially since the ozone absorption given is 
probably too high). 
It should be pointed out that the data of Fig. 5 may 
not be used with Beer’s law in the customary absorp- 
tion computations, since the bands are too wide to be 
considered as monochromatic; the empirical data of 
Fig. 5 must be utilized. However, for many purposes 
the use of Beer’s law will lead to practical results [63]. 
A few transparent regions exist in the far infrared. 
The region near 10 p is relatively transparent with re- 
gard to water-vapor absorption, and although very 
little solar energy is available in this region, it has been 
used to measure atmospheric ozone which has an ab- 
sorption band near 9.6 uw. Adel has found that the 17-24 yu 
region is also relatively transparent. 
Total Water-Vapor Absorption. In order to determine 
the temperature change in the atmosphere produced by 
absorption of radiation by water vapor, a summation of 
absorption over all wave lengths is required, and such 
summations have been computed by several authors. 
For example, using Fowle’s absorption data, Kimball 
[52] computed the total absorption as shown in curve 
(16), Fig. 6. The curve shows the fraction of J) absorbed 
by water vapor as a function of the water vapor in the 
sun’s spectral path, that is, as a function of mw. By 
using data similar to these absorptions, Tanck [74] 
computed temperature rises of about 0.1-0.7 centi- 
grade degrees per day at Hamburg depending on the 
season and the height (up to 6 km). The order of magni- 
tude of the absorptions given by Tanck’s equation 
agrees with airplane measurements [29]. 
Clouds. Thanks to Fowle and to some recent studies 
[19, 24], the status of water-vapor absorption is fairly 
well known; however, the absorption of solar energy by 
clouds is comparatively unknown. A few theoretical 
estimates have been published [45], but very few meas- 
urements have been made. 
One method of measuring the amount of energy ab- 
sorbed is to measure simultaneously, with pyrheliom- 
eters, the energy leaving and entering the cloud layer 
both at its upper and lower surfaces; the difference be- 
tween the energy which enters the cloud and that 
which leaves it is the amount absorbed. Neiburger [66], 
using one blimp on which to mount two pyrheliometers, 
one facing upward and the other downward, made 
many vertical traverses through stratus clouds. When 
the blimp was below the clouds, he estimated the up- 
ward- and downward-flowing radiation at the top of 
the cloud. Because he lacked a second blimp, simul- 
taneous measurements could not be made both below 
and above the cloud, and since the albedos of clouds 
vary appreciably over short distances and times, errors 
may have been introduced because of the lack of simul- 
taneity. At any rate, Neiburger’s measurements, which 
were probably the first absorption measurements made, 
indicate that in the mean about 5 to 9 per cent of the 
incident radiation is absorbed in stratus clouds, and 
that there may be large variations from the mean. 
To measure the absorption in other types of clouds, 
especially over extensive cloud decks, the United States 
Weather Bureau, through the cooperation of the Air 
Force and the Office of Naval Research, has made 
measurements using B-29 airplanes as platforms for 
the two pyrheliometers. On a few occasions two air- 
planes, each equipped with two pyrheliometers, have 
been flown, one vertically above the other, with the 
cloud deck between them. When only one airplane was 
available, the plane, carrying two pyrheliometers, was 
flown above the cloud deck and above a pyrheliometer 
which was located on the ground. In the absence of 
ground snow cover, a good estimate can be made of the 
albedo of the ground; or if the ceiling is not too low, 
the albedo of the ground can be measured by flying the 
airplane below the clouds. Preliminary results from 
these measurements indicate that the absorption by 
these deep widespread systems averages about 20 per 
cent of the solar radiation incident on the cloud.® These 
measurements are still few in number and should be 
verified by additional determinations. 
6. Reported by T. H. MacDonald at the January 1950 meet- 
ing of the American Meteorological Society in St. Louis, 
Missouri. 
