Geoscience Reference
In-Depth Information
same. It is clear from
Figure 2.13
that, taking an annual
figure, the tropics would receive the most radiation, as the
input never falls to low values, unlike the situation at the
poles, where twenty-four hours of daylight in summer
become twenty-four hours of darkness in winter.
(a)
Summer solstice 21 June
N
23.5°
ua
North
Pole
Endogenetic energy
In addition to the exogenetic energy, a minute proportion
of the total energy comes from within Earth and is thus
endogenetic
(Greek
endon
, 'within', +
genos
).
The most obvious source of endogenetic energy is the
hot interior of Earth. The outer core of the globe consists
of molten materials at immense pressures, and at tempera-
tures up to 2,600
23.5°
S
Equinoxes
N
u
a
Arctic Circle
Tropic of Cancer
C. There is an almost immeasurably
small and continuous conduction of this heat to the
ground surface that adds to the energy inputs acting on
the landscape. It can be detected in deep mines and caves.
Locally the decay of radioactive minerals can provide
energy to the surface. More dramatic leakages of this
endogenetic energy are seen in the form of volcanoes,
hot springs and various other tectonic activities. Taken
together, all sources of endogenetic energy appear to
contribute no more than 0·0001 per cent of the total
energy supply averaged over Earth's entire surface, though
they may have been more important in the past during
periods of crustal instability.
Equator
North
Pole
Tropic of Capricorn
Antarctic Circle
S
Winter solstice 22 December
N
23.5°
ua
North
Pole
23.5°
S
Nature of Earth's energy output
The output of energy from Earth is in radiant form, but
it is not identical to the input of radiant energy from the
sun. Earth has modified the input by a variety of processes.
Some of the original solar energy input is reflected by
clouds or the ground surface and returned to space with
little change in its radiative properties; it is still short-wave
radiation. As insolation passes through the atmosphere
it is scattered, much of it towards Earth, but a small
proportion goes back to space as an output of short-wave
energy.
Of greatest importance is the emission of radiant
energy from Earth itself. As a result of the absorption of
solar energy in the atmosphere and at the surface, Earth
will have gained energy that will be converted into heat.
In turn, Earth and its atmosphere emit radiation following
Wien's law. The average temperature of Earth is about
290K, while that of the atmosphere is a chilly 250K.
Consequently the energy emission will reach a maximum
(b)
Zenith
22 March/22 Sept.
Zenith
21 June
21 Dec.
21 June
26.5°
23.5°
23.5°
22 March/22 Sept.
21 Dec.
63.5°
40°
16.5°
50°
N
Equator
(1)
SS
N
(2)
the solstices and the equinoxes; (b) position of the midday sun
(1) at the equator and (2) at 50°N at the solstices and the
equinoxes.
