28 RADIATION 
cent, which is the calculated reflection for the whole 
ultraviolet radiation (A < 0.4 y). 
Several other estimates of A have been computed by 
assuming some value for the average reflection by 
clouds and/or by studying the average transmission to 
the earth’s surface and estimating the amount of energy 
absorbed by the atmosphere and clouds. Notable among 
these calculations of A are the 43 per cent of Aldrich [7] 
and the 41.5 per cent of Baur and Philipps [14]. 
In addition to the average value of the earth’s albedo, 
Danjon found large fluctuations of the albedo from 
season to season and among his individual measure- 
ments. Some average values of the visual albedo varied 
from 30 per cent in August to 50 per cent in October. 
This large variation is not too surprising. Nor does 
absence of a relation either to snow cover or cloud 
amount cast serious doubt on this albedo variability. 
The amount of solar energy incident on the snow- 
covered areas is small so that they contribute little to 
the total albedo. As for cloud amount, even if this latter 
quantity were known on a world-wide basis, it would 
not uniquely determine the contribution of clouds to 
the earth’s albedo because the albedos of individual 
cloud types themselves are extremely variable. 
The meteorological significance of this large albedo 
variation is obvious. If there is any possibility, as 
several authors believe, that variations in the solar 
constant of about 1—2 per cent could influence the state 
of the weather, then the much larger changes in the 
reflected energy and hence in the absorbed energy must 
be significant indeed. Only the absorbed energy can 
affect the atmosphere. Perhaps short-period changes in 
the solar constant cause changes in the earth’s albedo, 
but such a causal relationship is not obvious a priori 
and would have to be established before it could be ac- 
cepted. At any rate, whatever the cause of its change, 
measurements of the earth’s albedo would indicate the 
large changes in the solar energy which is absorbed and 
potentially becomes available for meteorological proc- 
esses. 
The best way to measure the albedo, as with other 
solar-variation effects, is to mount instruments on a 
satellite stationed outside the earth. In the absence of 
such a satellite, techniques similar to Danjon’s must 
suffice and his results will have to be verified. Danjon’s 
technique would require a little elaboration. For ex- 
ample, from measurements in France, reflections from 
the Pacific Ocean area would not influence the moon’s 
dark-side illumination. Hence, measurements from 
widely separated regions would be needed to determine 
the albedo of the whole earth. Also, as stated pre- 
viously, an estimate of the ultraviolet and infrared 
albedo would also be required. However, the main 
changes in albedo from day to day would be given by 
visual albedos alone if the corrections should prove 
difficult to obtain. 
Solar Energy at the Earth’s Surface. We have con- 
sidered the depletion of solar energy through scattering 
and absorption by the various constituents of the at- 
mosphere. The remainder of the direct radiation reaches 
the ground, and of course, a considerable portion of the 
scattered radiation also arrives at the ground as radia- 
tion from either clear or cloudy skies. 
Cloudless Sky. Figure 6 shows the fractional trans- 
mission at normal incidence to the sun as a function of 
air mass and of water-vapor content for a moist, dust- 
less atmosphere. Multiplication by the solar constant 
will give the transmission in absolute units. For many 
purposes it will be of greater interest to determine the 
cloudless-sky energy Q) which reaches the horizontal 
La 
At 
12 
aq O. 
1.0 2.0 
AIR MASS (mp) 
PRECIPIT. 
Fie. 9.—Intensity of solar radiation on a horizontal surface 
(ly min™) as a function of air mass m, and precipitable water 
vapor w for a cloudless, dustless atmosphere. For elevated 
stations where pressure is p in millibars, multiply radiation by 
Wee Dashed lines represent extrapolated values. (After 
Fritz [31].) 
ground. Figure 9 shows the radiation on a horizontal 
surface during dust-free conditions, as a function of 
optical air mass and precipitable water-vapor content. 
The computations are based on Fig. 6 and a combina- 
tion of equations from Klein [81, 53]; it is assumed that 
half the scattered radiation reaches the ground. Much 
of the scattered radiation does reach the ground, but 
obviously this final result is only an approximation 
which agrees fairly well with observations and with 
other calculations. Summations over 24 hours (with the 
aid of Fig. 9) furnish daily totals which, when cor- 
rected by comparison with observations at several lo- 
cations, show the geographic and seasonal variation of 
cloudless-day radiation over large areas [31]. Such com- 
putations indicate that in the United States about 80 
per cent of the incident extraterrestrial energy reaches 
the ground during cloudless days. 
Some variations from average values are naturally 
to be expected. In cities particularly, one would expect 
marked average decreases of the order of about 20 per 
cent [40]. For snow-covered terrain, increased radiation 
will be measured because the strongly reflected radia- 
tion will be partially scattered back to the ground by 
the atmosphere. 
Transmission through Clouds. Clouds, however, will 
generally introduce the largest variations of the solar 
energy which reaches the ground. Haurwitz [43] has 
given the average solar energy transmitted to a hori- 
zontal surface through various types of clouds at Blue 
Hill, Massachusetts, and also the percentage of cloud- 
less-sky radiation transmitted. His data are shown in 
Fig. 10 and in Table VII. Large variations about these 
