Geoscience Reference
In-Depth Information
Table 3.1 Daily solar radiation on a horizontal surface outside the atmosphere: W m -2 .
Date
90°N70
50
30
0
30
50
70
90°S
21 Dec
0
0
86
227
410
507
514
526
559
21 Mar
0
149
280
378
436
378
280
149
0
22 June
524
492
482
474
384
213
80
0
0
23
Sept0
147
276
373
430
372
276
147
0
Source : After Berger (1996).
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
theoretically greater than that received by the northern
hemisphere during the June solstice, due to the previ-
ously 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.
90˚S
60˚S
30˚S
30˚N
60˚N
90˚N
90˚S
60˚S
30˚S
Equinox
Solstice
30˚N
Equinox
60˚N
90˚N
Solstice
Figure 3.4 The variations of solar radiation with latitude and
season for the whole globe, assuming no atmosphere. This
assumption explains the abnormally high amounts of radiation
received at the poles in summer, when daylight lasts for twenty-
four hours each day.
Source : After W. M. Davis.
B SURFACE RECEIPT OF SOLAR
RADIATION AND ITS EFFECTS
1 Energy transfer within the
earth-atmosphere system
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 sol-
stice, the sun's altitude is a constant 23 1 2 ° throughout
the day at the North Pole and the sun is directly overhead
at noon at the Tropic of Cancer (23 1 2 °N).
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 because of the effect of
the atmosphere on energy transfer. Heat energy can be
transferred by three mechanisms:
1 Radiation : Electromagnetic waves transfer energy
(both heat and light) between two bodies, without
the necessary aid of an intervening material medium,
at a speed of 300
4 Length of day
The length of daylight also affects the amount of
radiation that is received. Obviously, the longer the time
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 these factors produces
the pattern of receipt of solar energy at the top of the
10 6 m s -1 (i.e. the speed of light).
This is so with solar energy through space, whereas
the earth's atmosphere allows the passage of radi-
ation only at certain wavelengths and restricts that at
others.
Radiation entering the atmosphere may be
absorbed in certain wavelengths by atmospheric
gases but, as shown in Figure 3.1, most short-wave
radiation is transmitted without absorption. Scattering
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