ABSORPTION.Earth and its atmosphere
absorb about 64 percent of the insolation. Land and
water surfaces of Earth absorb 51 percent of this
insolation. Ozone, carbon dioxide, and water vapor
directly absorb the remaining 13 percent. These gases
absorb the insolation at certain wavelengths. For
example, ozone absorbs only a small percentage of the
insolation. The portion or type the ozone does absorb is
critical since it reduces ultraviolet radiation to a level
where animal life can safely exist. The most important
absorption occurs with carbon dioxide and water vapor,
which absorb strongly over a broader wavelength band.
Clouds are by far the most important absorbers of
radiation at essentially all wavelengths. In sunlight
clouds reflect a high percentage of the incident solar
radiation and account for most of the brightness of
Earth as seen from space.
There are regions, such as areas of clear skies,
where carbon dioxide and water vapor are at a
minimum and so is absorption. These areas are called
atmospheric windows and allow insolation to pass
through the atmosphere relatively unimpeded.
The atmosphere conserves the heat energy of Earth
because it absorbs radiation selectively. Most of the
solar radiation in clear skies is transmitted to Earths
surface, but a large part of the outgoing terrestrial
radiation is absorbed and reradiated back to the surface.
This is called the greenhouse effect. A greenhouse
permits most of the short-wave solar radiation to pass
through the glass roof and sides, and to be absorbed by
the floor, ground or plants inside. These objects
reradiate energy at their temperatures of about 300°K,
which is a higher temperature than the energy that was
initially received. The glass absorbs the energy at these
wavelengths and sends part of it back into the
greenhouse, causing the inside of the structure to
become warmer than the outside. The atmosphere acts
similarly, transmitting and absorbing in somewhat the
same way as the glass. If the greenhouse effect did not
exist, Earths temperature would be 35°C cooler than
the 15°C average temperature we now enjoy, because
the insolation would be reradiated back to space.
Of course, the atmosphere is not a contained space
like a greenhouse because there are heat transport
mechanisms such as winds, vertical currents, and
mixing with surrounding and adjacent cooler air.
RADIATION (HEAT) BALANCE IN THE
The Sun radiates energy to Earth, Earth radiates
energy back to space, and the atmosphere radiates
energy also. As is shown in figure 1-7, a balance is
maintained between incoming and outgoing radiation.
This section of the lesson explains the various radiation
processes involved in maintaining this critical balance
and the effects produced in the atmosphere.
We have learned that an object reradiates energy at
a higher temperature. Therefore, the more the Sun heats
Earth, the greater the amount of heat energy Earth
reradiates. If this rate of heat loss/gain did not balance,
Earth would become continuously colder or warmer.
Terrestrial (Earth) Radiation
Radiation emitted by Earth is almost entirely
long-wave radiation. Most of the terrestrial radiation is
absorbed by the water vapor in the atmosphere and
some by other gases (about 8 percent is radiated
directly to outer space). This radiant energy is
vertically. Horizontal flux (flow or transport) of energy
need not be considered due to a lack of horizontal
temperature differences. The vertical, upward or
downward, flux is of extreme significance.
Convection and turbulence carry aloft some of this
(hydrological cycle), carries the remainder into the
The atmosphere reradiates to outer space most of
the terrestrial radiation (about 43 percent) and
insolation (about 13 percent) that it has absorbed. Some
of this reradiation is emitted earthward and is known as
counterradiation. This radiation is of great importance
in the greenhouse effect.
Heat Balance and Transfer in the Atmosphere
Earth does not receive equal radiation at all points
as was shown in figure 1-4. The east-west rotation of
Earth provides equal exposure to sunlight but latitude
and dispersion do affect the amount of incident
radiation received. The poles receive far less incident
radiation than the equator. This uneven heating is called