SOLAR RADIANT ENERGY 29 
average values are of course to be expected for indi- 
vidual cases. 
The effect of snow in increasing the solar radiation 
will be particularly pronounced for overcast conditions 
because the energy which is reflected by the snow will 
be strongly reflected towards the ground by the base of 
the cloud. This is shown for visible radiation in Fig. 11 
[49]. For wider application the data of Table VII and 
CAL GM 2 HRT! 
1.0 2.0 
3.0 
MASS 
4.0 5.0 
AIR 
Fre. 10—Solar radiation on a horizontal surface through 
the types of clouds indicated. (After Hawrwitz [43].) 
Taste VII. Rarro or Sonar RADIATION WITH OVERCAST SKIES 
TO SOLAR RADIATION FOR CLOUDLESS SKIES 
(in per cent) 
(After Haurwitz [43]) 
Air mass Ci Gs | Ac As Sc St Ns Fog 
ilgal 85 84 52 41 35 25 15 17 
1.5 84 81 51 41 34 25 17 17 
2.0 84 78 50 4] 34 25 19 17 
2.5 83 74 49 4] 33 25 21 18 
3.0 82 71 47 41 32 24 25 18 
3.5 $1 68 46 41 31 24 18 
4.0 80 65 45 41 31 18 
4.5 30 19 
5.0 29 19 
Fig. 10 should be separated for conditions of snow- 
covered and snow-free ground. Also similar data should 
be computed for areas in which the climate is different 
from that of Blue Hill. 
Average Conditions. We turn now to the estimation 
of the average solar radiation Q which reaches the 
earth’s horizontal surface on the average for all days. 
It is commonly assumed that 
Q = Qi (a + BS) (24) 
will give average values of Q; here a and b are constants 
such that a + b = 1, S is the percentage of possible 
sunshine, and @p is the value of Q when S = 100 (clear 
skies). This was really already assumed implicitly by 
Kimball in 1919 [51]. From examination of the radiation 
on individual days, average values of a have been 
found for S = 0 and/or for c = 10 (overcast) by many 
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Fic. 11.—Diffuse visible skylight for different types of 
clouds (10/10 sky cover) above ground with snow cover 
(hatched bars) and without snow cover (clear bars) for solar 
altitudes of 7° and 30°. Dashed horizontal lines apply for clear 
skies. (After Kalitin [49].) 
observers. Kimball found a = 0.22 for Washington, 
D. C., and obtained 
Q/Q = (0.22 + 0.788). (25) 
Angstrom found a = 0.235 for Stockholm; for annual 
values at Blue Hill, Haurwitz found a = 0.22 or 0.30, 
depending on the assumptions; Mosby obtained a = 
0.54 for the Arctic [42]. Recently several other authors 
have found values of a which vary considerably. Never- 
