SOLAR RADIANT ENERGY 19 
that this is not the case, but that the data are inde- 
pendent. It should be pointed out, however, that ques- 
tions regarding the accuracy of data from all solar- 
constant stations except Montezuma have been raised 
from time to time by Abbot and his colleagues, es- 
pecially regarding the data prior to 1920 [8]. 
With regard to the seasonal variation of solar con- 
stants, Abbot has made comparisons of differences be- 
tween solar-constant measurements at Southern Hemi- 
sphere and Northern Hemisphere stations on a monthly 
basis and finds no seasonal variation in the differences. 
The season being opposite in the two hemispheres, this 
would indicate lack of seasonal variation in Ip. Although 
he found no twelve-month period in Jo variations, 
Abbot has found fourteen other different periods in the 
solar-constant variations. Paranjpe questioned the ex- 
istence of these periods, but Sterne [73], while sup- 
porting Abbot’s claim as to the statistical significance 
of some of his periods, found a strongly significant 
period of twelve months, suggesting a possible terres- 
trial effect. 
We see therefore that there has been considerable 
controversy regarding the reality of the variations 
shown by the solar-constant measurements, and nu- 
merous additional arguments, pro and con, could be 
unearthed. But the present state of affairs can be 
summed up as follows: The energy emitted by the sun 
does change from time to time. This is indicated by the 
changes which can be seen in the sun’s surface and by 
the changes revealed by the ionospheric and radio 
measurements. However, the percentage change in the 
total energy output is small, amounting at most to 1 
or 2 per cent [3], and whether the solar-constant meas- 
urements as observed from the earth’s surface are 
sufficiently accurate to reveal the time and/or magni- 
tude of such variations is still a debatable question. 
This controversy may be resolved in several ways. 
The surest way would be to make the measurements 
from a satellite stationed outside the earth’s atmo- 
sphere. If rockets could be adequately equipped, fre- 
quent measurements from them would also be 
satisfactory. Perhaps balloons may serve for this pur- 
pose. But if these methods are economically or experi- 
mentally remote, the establishment of a few additional 
surface stations might help. Im science, important 
results ordinarily are not finally accepted until they 
have been corroborated by independent observers. Here 
too, if the questions raised as to the amount and time 
variations of the solar constant are to be answered, any 
additional stations should be operated by independent 
observers, as was long ago suggested by Marvin [58]. 
However, there is a more important parameter to 
measure than the variation of Jo; that parameter is 
the earth’s albedo. From the meteorological viewpoint, 
the predominant interest is not in J) variations as such, 
but in the amount of energy absorbed by the earth and 
its atmosphere, and in the variations of this amount. 
As will be shown later, the solar energy reflected by the 
planet Earth varies so much that the energy absorbed 
may vary by +15 per cent or more from the mean 
absorbed energy. This is to be compared with a pos- 
sible change of --1 per cent in the solar constant. 
Extraterrestrial Solar Energy on a Horizontal Sur- 
face. Of fundamental importance for meteorology is 
the energy @ which reaches a horizontal area at the 
earth’s surface. For purposes of comparison and to 
permit certain computations of Q, Milankovitch [61] 
computed Qz, the extraterrestrial value of Q, from the 
relation 
tT 
On= | 10 eas, Zits (6) 
t 
nn 
where t; is the time of sunrise, f2 the time of sunset, Z 
the sun’s zenith distance, and p the radius vector of 
the earth. If we take J) = 1.94 ly min“, Qz, in langleys 
per day, is given in Fig. 3. 
(JAN. FEB. MAR. APRIL MAY JUNE JULY AUG. SEPT OCT. NOV 
“ET PM 
| j 
YY fo) 
nS A /} LE 
10 
Y we My a 
60 . & + if Fe =~ — 
ee WARS D, 
rs 2 
50 S } D 
Oo 
Les /\ 1 o| Lal 
# STE = Oo OF 
Zz = a Sn 
. PLS! al irl Oo | Bi 
30 Ke) a n- & a 
[ray 
ae jcaleas RO OF 4 \ N o| a 
y in © SY = > Te 
“ LAH] & Zz \Si a0} By 5 SJ@ = 
“Eq mie fl \al ot SI 
oO + f 
1 
ae 
Je i ! 
=20) = 
500) 
=30° + - 
8 
I) | 
[s) 1 1 
é|// 
-50°}-— > 
I 
a 
JAN FEB. 
CMM MALL | 
MAR. APRIL MAY JUNE JULY AUG. SEPT. OCT. NOV. DEC. 
Fia. 3.—Solar radiation on a horizontal surface outside the 
earth’s atmosphere (ly day). (After List [56].) 
TERRESTRIAL EFFECTS ON SOLAR RADIATION 
Having considered the amount and the spectral in- 
tensity of extraterrestrial solar energy, we turn now to 
the effect which the earth and its atmosphere have 
upon the incident radiation. In general the atmospheric 
elements absorb and scatter part of the incident solar 
energy. 
Absorption. At the outset it should be noted that » 
< 2900 A (approximately) is not observed at the 
ground; nearly all energy of ) < 2900 A is absorbed 
and a small part is scattered back to space by the gases 
of the atmosphere. 
The Ionosphere and Ozonosphere. Energy of \ < 
1000 A is highly absorbed by O, O2, or N»2 [62]; such 
energy is responsible for the ionization and heating of 
the ionospheric regions. For energy in the region from 
1300 A to 3500 A, Craig [21] has made a thorough study 
