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Energy and Radiation
Global Patterns of Insolation, Net radiation, and Heat
Patterns of insolation and net radiation which determine
the location of plants, animals, climate, soils and other elements of
our physical environment can be discerned from Figures 4.25 through
4.27. Figure 4.25 illustrates the latitudinal distribution of incoming
solar radiation and outgoing terrestrial radiation. From approximately 35o N to 35o S latitude
(the red area of the graph) there
is a surplus of energy as incoming radiation exceeds outgoing. The blue regions indicate that there
is more
outgoing energy than incoming, yielding a net loss of energy from
the Earth's surface. One might ask then why the middle to higher
latitudes aren't getting colder through time as a result of the net loss,
and the subtropical to equatorial regions getting constantly hotter due
to the net gain. The reason is that the energy is redistributed
by circulation of the atmosphere and oceans. Heat gained in the tropics is transported poleward by the global circulation of air and warm ocean currents to
heat higher latitude regions. Cooler air from the higher latitudes and
cold ocean currents push equatorward to cool the lower latitudes. This
process of redistributing energy in the Earth system helps maintain a
long-term energy balance.
Figure 4.25 Latitudinal Variation of the Radiation
Balance
Insolation
Figure 4.25 simplifies the geographic distribution of
insolation. For the Earth as a whole, particular patterns can be
accounted for by variation in surface features that impact insolation.
Insolation maxima are found in the tropical and subtropical deserts of
the earth. Here, high sun angles and a lack of cloud cover in desert
regions allow much solar radiation to the surface. Insolation decreases
to a
minimum at the poles where low sun angles and the fact that the Sun doesn't rise above the horizon
nearly half the year reduces annual
insolation.
Figure 4.26 Annual global distribution of solar radiation
(Kcal/cm2)
(after
Sellers, W. D., 1965)
Click image to enlarge
Net radiation
Net radiation exhibits a different pattern from that of
insolation. Maximum net
radiation is found over the tropical and subtropical oceans. The sun
angle is always high over the tropical oceans so the surface receives
intense radiation throughout the year. With an high sun angle the albedo
of the surface is low and absorption is high. However, the energy received is partitioned into warming the surface as
well as evaporating water. The result is a lower surface temperature
than one might expect with such high sun angles. Additionally, the high
specific heat of water means that it takes much more energy to heat a
unit mass of water than that of land, resulting in lower ocean surface temperatures.
With lower surface temperatures the water surface does not radiate as much longwave radiation out to the atmosphere
as nearby land at the same latitude. With much radiation
coming in and little going out, the net value is large compared to land at the same latitude. Net radiation is at a minimum over the
poles as the sunlight that comes in at a low angle is reflected from the
ice-covered surface. Combined with the long polar night, very little net radiation
is found at these latitudes.
Figure 4.27 Annual global distribution of net radiation
(Kcal/cm2)
(after
Sellers, W. D., 1965)
Click image to enlarge
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