SOLAR RADIANT ENERGY AND ITS MODIFICATION BY THE EARTH 
AND ITS ATMOSPHERE 
By SIGMUND FRITZ 
U. S. Weather Bureau, Washington, D. C. 
The sun is the principal source of the energy which, 
by devious means, becomes the internal, potential, and 
kinetic energy of the atmosphere. The solar irradiation 
of a unit horizontal surface at the outer limits of the 
earth’s atmosphere can be evaluated, at least in relative 
units, from astronomical and trigonometrical considera- 
tions [61]; thus on a relative scale, the diurnal and 
seasonal variations of this solar irradiation above the 
atmosphere are known. On the average, variations simi- 
lar to these occur also at the earth’s surface, and the 
associated diurnal and seasonal changes in atmospheric 
temperature are commonplace knowledge [52]. 
There are, however, additional changes in effective 
solar irradiation of the planet Earth which are super- 
posed on the trigonometrical variations. These are of 
two kinds. The first is due to the change in the quality 
and quantity of energy which leaves the sun. The second 
is caused by changes in the reflectivity of the atmo- 
sphere (including clouds) and of the earth’s surface; the 
solar energy which is immediately reflected to space 
cannot be meteorologically effective. 
In contrast to the regular, astronomically induced 
changes in meteorological parameters (notably diurnal 
and seasonal atmospheric temperature changes), the 
large-scale meteorological consequences of these irregu- 
lar changes in solar irradiation are far from obvious, 
if, indeed, any such meteorological effects can be shown 
to be induced at all by them. For example, changes of 
the first kind (7.e., in solar output) have been invoked 
as possible causes of abnormal heating in the ozone 
layer with subsequent pressure changes at the earth’s 
surface [44]; changes of the second kind (7.e., in reflec- 
tion by the earth or clouds) are important, for instance, 
in local turbulence of the air near the ground, but are 
rarely used to explain widespread meteorological phe- 
nomena. These as well as other solar-induced meteor- 
ological phenomena are discussed elsewhere in this 
Compendium. In this article we shall, for the most part, 
examine the solar energy itself and shall mention its 
meteorological effects only incidentally. 
SOLAR RADIATION OUTSIDE THE EARTH’S 
ATMOSPHERE 
The Sun. The sun, located about 93,000,000 miles 
from the earth, is a large, hot, gaseous mass. When 
viewed through a smoked glass it appears as a smooth 
circular disk which is called the photosphere, but when 
examined with the aid of more refined techniques, the 
photospheric surface appears highly granulated and is 
surrounded by a gaseous envelope which is commonly 
divided into three layers for descriptive purposes. Of 
these the one just outside the photosphere is the rela- 
13 
tively thin reversing layer, so called because the spectral 
lines ordinarily seen as dark absorption lines in the 
photospheric spectrum appear as bright emission lines 
when examined in the reversing layer; still farther from 
the photosphere is the chromosphere; and beyond that 
is the corona [4]. The sun’s “surface” and its surround- 
ing atmosphere are by no means static. The presence 
of short-lived photospheric grains, dark sunspots, bright 
areas (faculae and flocculi), and erupting prominences 
indicate that the entire observable sun is in a state of 
considerable turmoil. This turmoil is associated with 
variations in the quantity and spectral intensity dis- 
tribution of the solar energy which subsequently irra- 
diates the outer limits of the earth’s atmosphere. 
Average Spectral Distribution of Sunlight. To calcu- 
late the solar energy available for meteorological proc- 
esses, it is desirable to measure the amount of solar 
energy I) (the so-called solar constant) which reaches 
the outer atmosphere of the earth. To determine the 
interaction between our atmosphere and the sun’s 
energy, the distribution of spectral imtensity In of the 
extraterrestrial solar energy is also required. For both 
of these quantities we are largely indebted to the Astro- 
physical Observatory of the Smithsonian Institution, 
whose determinations of J) and J), are monuments to 
the work of the Observatory. 
Until recently, the sun’s energy had not been directly 
observed below wave lengths of about 2900 A because 
of the absorption of energy of shorter wave lengths by 
the envelope of ozone which surrounds the earth up to 
about 50 km. For wave lengths greater than 2900 A, 
selective scattering and absorption, mainly by air, dust, 
and water vapor, modify the solar spectrum; these 
troublesome modifications must be eliminated from 
measurements made at the ground under cloudless skies 
to obtain the extraterrestrial spectrum. In the region 
from about 0.29 pv to 2.5 w numerous measurements and 
extrapolations have been made. The Smithsonian Insti- 
tution [3] is the main source of spectral information for 
the region above 3400 A, but its latest summary of data 
was compiled in 1923 [2]. Recently, V-2 rocket observa- 
tions have extended the measured spectrum down to 
2200 A [47]. : 
Visible and Near Infrared Radiation. Both the total 
amount and the spectral distribution of the radiant 
energy emitted by a black body are determined by its 
temperature. It is therefore convenient to describe the 
radiant energy from a source in terms of the black-body 
temperature which would most nearly produce the ob- 
served energy. The sun does not radiate as a black 
body. Despite this fact, the average observed energy 
curve from about 0.45 « to about 2.0 » closely approxi- 
