Geoscience Reference
In-Depth Information
surface outside the atmosphere are given in
Table
3.1
. The intensity on a horizontal surface (
I
h
) is
determined from:
the time that the sun shines the greater is the
quantity of radiation that a given portion of the
earth will receive. At the equator, for example, the
day length is close to 12 hours in all months,
whereas at the poles it varies between 0 and 24
hours from winter (polar night) to summer (see
Figure 3.3
).
The combination of all of these factors
produces the pattern of receipt of solar energy at
the top of the atmosphere shown in
Figure 3.4
.
The polar regions receive their maximum
amounts of solar radiation during their summer
solstices, which is the period of continuous day.
The amount received during the December
solstice in the Southern Hemisphere is theoret-
ically greater than that received by the Northern
Hemisphere during the June solstice, due to the
previously mentioned elliptical path of the earth
around the sun (see
Table 3.1
). The equator has
two radiation maxima at the equinoxes and two
minima at the solstices, due to the apparent
passage of the sun during its double annual
movement between the Northern and Southern
Hemispheres.
I
h
=
I
0
sin
d
where
I
0
= the solar constant and
d
= the angle
between the surface and the solar beam.
3
Altitude of the sun
The altitude of the sun (i.e., the angle between
its rays and a tangent to the earth's surface at the
point of observation) also affects the amount of
solar radiation received at the surface of the
earth. The greater the sun's altitude, the more
concentrated is the radiation intensity per unit
area at the earth's surface and the shorter is the
path length of the beam through the atmosphere,
which decreases the atmospheric absorption.
There are, in addition, important variations with
solar altitude of the proportion of radiation
reflected by the surface, particularly in the case of
a water surface (see B.5, this chapter). The
principal factors that determine the sun's altitude
are, of course, the latitude of the site, the time of
day and the season (see
Figure 3.3
). At the June
solstice, the sun's altitude is a constant 23 1⁄2
B SURFACE RECEIPT OF
SOLAR RADIATION AND ITS
EFFECTS
°
throughout the day at the North Pole and the sun
is directly overhead at noon at the Tropic of
Cancer (23 1⁄2°N).
1 Energy transfer within the
earth-atmosphere system
So far, we have described the distribution of solar
radiation as if it were all available at the earth's
surface. This is, of course, unrealistic owing to the
4 Length of day
The length of daylight also affects the amount of
radiation that is received. Obviously, the longer
Table 3.1
Daily solar radiation on a horizontal surface outside the atmosphere: W m
-2
.
Date
90°N
70
50
30
0
30
50
70
90°S
Dec 21
0
0
86
227
410
507
514
526
559
Mar 21
0
149
280
378
436
378
280
149
0
June 22
524
492
482
474
384
213
80
0
0
Sept 23
0
147
276
373
430
372
276
147
0
Source: After Berger (1996).
