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 2 -
Globally Averaged Energy Budget
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Outgoing
Terrestrial Radiation
- The earth's surface, atmosphere, and clouds emit radiation
in the infrared band and near-infrared band.
- Outgoing infrared (IR) radiation from the earth's surface
(also called terrestrial radiation) is selectively absorbed
by certain molecules, particularly water vapor and carbon
dioxide.
Gases which absorb IR radiation
are termed collectively as "greenhouse
gases".
- Water vapor and carbon dioxide emit infrared radiation.
Infrared radiation from greenhouse
gases in the atmosphere is emitted in all directions,
including back to the earth's surface. It is this
re-emission to the earth's surface that maintains
a higher temperature on our planet than what would
be possible without the atmosphere.
- Condensed water is also an efficient absorber and emitter
of IR radiation. Thus, clouds act in a manner similar
to greenhouse gases.
- Satellite infrared imagery detects infrared emission
from clouds and the earth's surface and can be used during
both day and night.
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Global Energy
Balance
- When averaged over a year, the incoming energy in both
the earth and its atmosphere equals the outgoing energy.
If we consider the entire
Earth-atmosphere system, then the amount of radiation
entering the system must equal to the amount leaving,
or the system would continually heat or cool. Not
all of this energy is radiative energy; some is
sensible and latent heat.
- If we consider the atmosphere alone, we find that the
atmosphere experiences radiative cooling.
The atmosphere is kept from
a net cooling by the addition of energy by latent and sensible
heating.
- The atmosphere has a warming effect on Earth's surface
-- the "atmospheric greenhouse effect".
If Earth had no atmosphere, the globally
averaged surface temperature would be -18 degrees Celsius.
Because Earth does have an atmosphere, the average surface
temperature actually is 15 degrees Celsius.
The
atmosphere acts as a greenhouse because of gases that selectively
allow solar radiation to pass through but absorb and then re-emit
terrestrial radiation. These gases are collectively called "greenhouse gases" and
include water vapor, carbon dioxide, ozone, molecular oxygen,
methane and nitrous oxide. These gases are selective as to
which wavelengths they will absorb. For example, ozone absorbs
shortwave ultraviolet radiation whereas water vapor absorbs
infrared radiation more readily.
- Most of the sun's radiation that passes through the
atmosphere to hit the earth is in the visible part of
the spectrum.
- Most of the earth's radiation that escapes the atmosphere
is in the infrared band between 8 microns and 11 microns.
This
region of the spectrum is called the "atmospheric window".
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