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Earth's Energy Budget
Part 1
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Principle
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Absorption and re-emission of radiation
at the earth's surface is only one part of an intricate web
of heat transfer in the earth's planetary domain. Equally
important are selective absorption and emission of radiation
from molecules in the atmosphere. If the earth did not have
an atmosphere, surface temperatures would be too cold to
sustain life. If too many gases which absorb and emit
infrared radiation were present in the atmosphere, surface
temperatures would be too hot to sustain life.
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Figure 1 - Radiation "Budget" for Incoming Solar
Radiation
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Incoming Solar Radiation
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- The sun radiates mostly in the visible band, but also
in the ultraviolet (shorter wavelength).
- When averaged globally and annually, only 51% of the
solar radiation striking the earth and its atmosphere is
absorbed at the surface.
The atmosphere absorbs 19% of
incoming solar radiation and the remaining 30% is
reflected back into space.
- The shortest wavelengths of solar radiation (those
most dangerous to life) are absorbed by molecules in the
upper and middle atmosphere.
In particular, ozone selectively
absorbs ultraviolet radiation while allowing visible
radiation to pass through relatively unhindered.
- Reflection significantly affects the solar radiation
that reaches the ground, as the sun's rays could be
reflected off of air molecules (termed scattering),
clouds or the ground itself.
Light-colored or shiny objects
reflect more radiation than dark objects. Energy that is
reflected cannot be absorbed or transmitted through an
object.
- Different surfaces have different albedos (see Table
1).
Meteorologists refer to the
percent of radiation returning from a surface compared to
the incident radiation as the "albedo". For example, the
earth reflects an average of 30% of the incoming
radiation, so the average albedo is 30%, or 0.3. Most of
the earth's average reflection results from
clouds.
- Visible satellite imagery can be used only during the
day, when visible solar radiation is striking the earth's
atmosphere and surface.
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Table 1- Average Reflectivity of Surfaces to Solar
Radiation
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Surface
Type
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%
Reflected
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Albedo
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fresh snow
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~90%
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~0.9
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thick clouds
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~80%
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~0.8
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thin clouds
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~40%
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~0.4
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ice
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~35%
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~0.35
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soil
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~20%
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~0.2
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forest
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~5%
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~0.05
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ocean (high sun angle)
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~5%
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~0.05
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ocean (low sun angle)
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~95%
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~0.95
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