Geoscience Reference
In-Depth Information
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LONGITUDE
Figure 3.9 Earth's mean annual net radiation budget, from satellite observations between April 1979 and March 1987. Units
are W m -2 .
Source: After Kyle et al. (1993)
Q * = Q H + Q G + Q E
ENERGY BALANCE
where Q E is the energy used for evaporation from the soil
and transpiration from plants. This is a simplification, as
changes in heat storage can take place and a small amount
of energy is used in plant growth.
The energy transfer into the atmosphere is the final
component of the radiation imbalance between surface
and atmosphere. The net radiational loss in the atmos-
phere is counteracted by this heat transfer from the
surface. So, over a long period, the atmosphere gains as
much energy as it loses.
The significance of sensible heat in the local energy
budget depends upon the frequency and intensity of
surface heating. Where the surface is usually hotter than
the air, values may be high, but where there is little
temperature difference, as over most oceans, sensible heat
transfer will be low. Estimates of its value are shown in
Figure 3.10 .
Uses of available energy
In the previous section we showed how Earth's surface
normally receives a surplus of radiation which leads to
warming there. This situation cannot last indefinitely, or
the temperature gradient in the air and the soil would
become enormous. Energy tends to flow down a gradient,
and as radiation is absorbed by the surface, so heat is
transmitted into the soil and into the air. This takes
place in proportion to the amount of energy originally
absorbed. We can express this mathematically as:
Q * = Q H + Q G
where Q * is the net radiation, Q H is the sensible heat
transfer into the air and Q G is the heat flow into or out of
the soil. If the surface is damp some of the energy will be
used in evaporation. Therefore:
 
 
 
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