SOLAR RADIANT ENERGY 27 
Such a computation [80], based on lunar measure- 
ments (to be described later), indicates that the average 
albedo of clouds is about 50-55 per cent of the energy 
incident on the clouds. This value may be compared 
with several series of measurements given in Table V 
and in Fig. 8. 
Tt will be noted that 80 per cent seems to be a very 
high value, and that the 78 per cent which Aldrich [7] 
measured over a single stratus cloud is not a suitable 
average. According to Bullrich [18], for average clouds 
of infinite thickness the albedos by cloud types will be 
as follows: stratocumulus, 0.78; stratus, 0.74; and alto- 
stratus, 0.46. A low albedo (39 to 59 per cent) for the 
altostratus type of cloud can be seen in the measure- 
ments in Table V. 
Albedo of the Earth’s Surface. The albedo of the 
ground has a very wide range of values also. In general, 
the lowest albedos occur over water. Forests or other 
configurations which can trap solar energy act nearly 
as black bodies and therefore likewise have low albedos. 
On the other hand, snow-covered terrain has a very 
high albedo; values up to 90 per cent have been meas- 
ured over fresh snow. As the snow becomes older or 
turns to ice, its albedo decreases markedly, and it 
absorbs more energy. Data for several types of terrain 
are given in Table VI. It will be noted that general 
terms such as “grass” give only an approximate esti- 
mate of the albedo, so that for investigation of such 
quantities as local turbulence near the ground, the 
albedo at the time and near the place in question will 
have to be measured. 
Taste VI. AtBgEDO or VARIOUS SURFACES 
(After List [56]) 
Surface Albedo (per cent) 
IDGEGI. c cld.cle 0.6 REACTOR eae ee 24-28 
elds) various types.........2.. les... vals 3-25 
CIEE SUMO E GTN Sete eis fafe oy eee veyareoet Syacays susyels 3-10 
Grass, various conditions................. 14-37 
GTOUMOM Aner I. ne ee nee 7-20 
iMilallal, Tolle lose Se hacises atts ceeiae oe ine emery eee 8-14 
Sand, CHAP, 15 SOS Rano ee ae eens 18 
SCM VGUMME PIT We cents canete nee sek wine 9 
SUOMI OTE COME IK ey. ii Neca tania) MELE. 46-86 
2.0 
2.1 
2.5 
3.4 
6.0 
13.4 
34.8 
58.4 
100.0 
_*For reflection of sun plus sky radiation Angstrém [10] 
gives: 
4 Cea heaceaeeeee 43.0 46.9 70.5 77.9 84.5 
A (per cent)....... 5.7 13.8 30.0 46.5 
The albedo of large water surfaces depends very 
largely on the angle of incidence of the light, or the 
sun’s zenith distance Z. Under cloudless conditions, the 
direct solar radiation follows Fresnel’s law of reflection 
very closely even when the wind is 10 mph [10]. For 
small values of Z (sun overhead) the reflectivity is very 
low, and it is not until the angle reaches about 65° 
that the albedo for solar and sky radiation becomes as 
much as 10 per cent. Under overcast skies the albedo 
of the sea surface is about 10 per cent, but changes a 
little even then with solar altitude and cloud thickness 
[65]. When the wind velocity is large enough to cause 
whitecaps, the albedo of the water increases, and Brooks 
[17, p. 460] gives 31 per cent as the albedo when the 
water surface is rough. Angstrém states, however, that 
in ordinary geophysical problems the data of Table 
VI are applicable. 
The low albedo of water and, in general, of land 
means that the contributions of the earth’s surface to 
the total albedo of the planet Earth must be small. 
As a rule, areas which are snow-covered receive little 
sunlight; the same may be said for water areas which 
have a high albedo (low solar altitudes). The net result 
is that, when the average cloudiness is considered, the 
earth’s surface contributes less than 4 per cent of the 
incident energy to the total albedo of the earth; this 
contribution is probably between 2 and 3 per cent [30]. 
The Albedo of the Planet Earth. The sum of the al- 
bedos of the various entities is the albedo A of the 
planet Earth. As stated earlier, smce there is a vari- 
ability of the albedo of clouds, and since clouds con- 
tribute most to the albedo of the planet Earth, A can 
be found only by measurements from or on bodies out- 
side the earth. 
Danjon [23] has made such measurements by viewing 
the moon with a suitable photometer. The moon is 
illuminated by two sources of light. The light side of 
the moon is illuminated directly by the sun; the dark 
side, by the earth. The light from the earth is swnlight 
which has been reflected by the earth to the dark side 
of the moon. Consequently, the brightness on the dark 
side of the moon, as compared with the brightness on 
the light side, is a measure of the sunlight reflected by 
the earth, that is, it is a measure of A. Of course, such 
measurements involve many difficulties, in addition to 
observational ones. For example, the spectral distribu- 
tion of the earth’s light is different from that of sun- 
light and doubtless changes with the albedo itself. 
Hence, any selective reflectivity by the moon would 
introduce errors. Another problem is the stellar magni- 
tude of the moon. When his paper was already in press, 
Danjon learned of a new value of the moon’s brightness 
which would somewhat alter his calculations. However, 
the value which Danjon originally used for the moon’s 
brightness is still quoted in some astronomy textbooks, 
so his calculations may be correct. Danjon’s average 
value of the earth’s albedo for visible light (about 0.4 u 
to 0.74 y) is 39 per cent. But visible light comprises only 
about half, or even a little less, of the total extrater- 
restrial solar energy. Hence, to determine the total 
albedo, adjustments to his measured values must be 
made for the ultraviolet and infrared portions of the 
solar energy. Calculation of the corrections gives an 
albedo of about 28 per cent for the infrared, about 50 
per cent for the ultraviolet, and 35 per cent for the total 
sunlight. The reflection in some portions of the ultra- 
violet where ozone does not absorb energy, as at 0.36 u 
for example, may be considerably higher than 50 per 
