Radiation in the Atmosphere
may seem to be completely transparent to solar radiation, but in fact
there are dynamic interactions occurring constantly that result in a complex
and delicately balanced system crucial to the continuation of present
life forms on Earth. In this section we will discuss how solar radiation
is absorbed and reflected by the atmosphere and the Earth--and two resulting
phenomena that are crucial to the maintenance of life on Earth:
- The atmosphere
acts as a filter, absorbing and reflecting portions of the electromagnetic
spectrum, such as the ultraviolet region, that are harmful to humans
and other life forms.
- The atmosphere
provides a natural "greenhouse effect," maintaining the temperatures
and climates in which life forms on Earth have evolved to survive.
controls the amount of solar radiation reaching the surface of the earth,
and regulates the amount of radiation from the Earth escaping into space.
Even seemingly slight changes in the concentrations of certain gases could
upset the balance of reactions and be detrimental to life as we know it.
We will now explain these two exchanges in detail, and later discuss their
relevance to two major environmental problems: global climate change and
stratospheric ozone depletion.
demonstrate how the atmosphere affects incoming solar radiation, Figure
8 shows the solar radiation spectrum first at the top of the atmosphere
(A, in red), and again at sea level (B, in blue). The absorption of the
molecules shown in Figure 8 is discussed in some detail because it is
critical to maintaining some of the most important conditions for our
viability on Earth. Note that the solar spectrum shown here is the same
as a part of Figure 4, enlarged to show the details of absorption.
8: Solar spectrum (A) above the atmosphere and (B) near the
with some of the radiation having been absorbed by molecules in
Also depicted is a curve of the spectrum detectable by the eye (C).
figure shows that ozone (O3) absorbs ultraviolet. O2
absorbs ultraviolet as well as some visible and infrared. The ultraviolet
absorption properties of O3 and O2 are central to
the protective ozone layer described later. Water vapor (H2O)
absorbs highly in the range of 0.4 and 0.9 µm and again above 1.2
µm and also in the microwave region. The absorption property of
H2O in the microwave region is, of course, the basis for the
efficient cooking of foods by microwave ovens. CO2 has high
absorption around 1.4 µm and above.
molecules absorb energy, the absorbed energy may go into causing a chemical
change (as in cooking food in a microwave oven), or it may be re-emitted.
Often molecules re-emit energy at wavelengths longer than that at which
it was absorbed. Thus when molecules such as H2O and CO2
absorb visible or infrared light, they often re-emit it as longer wavelength
infrared. This has great importance in our climate as described later.
small black curve labeled C in Figure 8 approximates the sensitivity spectrum
of our vision, or which wavelengths the eye can detect, with our maximum
sensitivity by the green and yellow. This is why yellow light is used
for markings on the roads and as the warning light color in traffic lights.