Principle
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. |
Figure 1 -
Radiation "Budget" for Incoming Solar Radiation
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Incoming
Solar Radiation
- 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
| Surface Type |
% Reflected |
Albedo |
| fresh snow |
~90% |
~0.9 |
| thick clouds |
~80% |
~0.8 |
| thin clouds |
~40% |
~0.4 |
| ice |
~35% |
~0.35 |
| soil |
~20% |
~0.2 |
| forest |
~5% |
~0.05 |
| ocean (high sun angle) |
~5% |
~0.05 |
| ocean (low sun angle) |
~95% |
~0.95 |
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Part 2 |
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