The insolation received at the surface of Earth
depends upon the solar constant (the rate at which solar
radiation is received outside Earths atmosphere), the
distance from the Sun, inclination of the Suns rays, and
the amount of insolation depleted while passing
through the atmosphere. The last two are the important
Depletion of Solar Radiation
If the Suns radiation was not filtered or depleted in
some manner, our planet would soon be too hot for life
to exist. We must now consider how the Suns heat
energy is both dispersed and depleted. This is
reflection, and absorption.
DISPERSION.Earlier it was learned that
Earths axis is inclined at an angle of 23 1/2°. This
inclination causes the Suns rays to be received on the
surface of Earth at varying angles of incidence,
depending on the position of Earth. When the Suns
rays are not perpendicular to the surface of Earth, the
energy becomes dispersed or spread out over a greater
area (figure.1-6). If the available energy reaching the
atmosphere is constant and is dispersed over a greater
area, the amount of energy at any given point within the
area decreases, and therefore the temperature is lower.
Dispersion of insolation in the atmosphere is caused by
the rotation of Earth. Dispersion of insolation also takes
place with the seasons in all latitudes, but especially in
the latitudes of the polar areas.
incoming solar radiation is scattered or diffused by the
atmosphere. Scattering is a phenomenon that occurs
when solar radiation passes through the air and some of
the wavelengths are deflected in all directions by
molecules of gases, suspended particles, and water
vapor. These suspended particles then act like a prism
and produce a variety of colors. Various wavelengths
and particle sizes result in complex scattering affects
that produce the blue sky. Scattering is also responsible
for the red Sun at sunset, varying cloud colors at sunrise
and sunset, and a variety of optical phenomena.
Scattering always occurs in the atmosphere, but
does not always produce dramatic settings. Under
conditions all that can be seen are white clouds and a
whitish haze. This occurs when there is a high moisture
content (large particle size) in the air and is called
diffuse reflection. About two-thirds of the normally
scattered radiation reaches earth as diffuse sky
radiation. Diffuse sky radiation may account for almost
100 percent of the radiation received by polar stations
whereby a surface turns a portion of the incident back
into the medium through which the radiation came.
A substance reflects some insolation. This means
that the electromagnetic waves simply bounce back
into space. Earth reflects an average of 36 percent of the
wavelengths on a surface is known as its albedo. Earths
average albedo is from 36 to 43 percent. That is, Earth
reflects 36 to 43 percent of insolation back into space.
In calculating the albedo of Earth, the assumption is
made that the average cloudiness over Earth is 52
percent. All surfaces do not have the same degree of
reflectivity; consequently, they do not have the same
albedo. Some examples are as follows:
Upper surfaces of clouds reflect from 40 to 80
percent, with an average of about 55 percent.
Snow surfaces reflect over 80 percent of
incoming sunlight for cold, fresh snow and as low
as 50 percent for old, dirty snow.
Land surfaces reflect from 5 percent of
incoming sunlight for dark forests to 30 percent for
Water surfaces (smooth) reflect from 2 percent,
when the Sun is directly overhead, to 100 percent
when, the Sun is very low on the horizon. This
increase is not linear. When the Sun is more than
25°above the horizon, the albedo is less than 10
percent. In general, the albedo of water is quite low.
When Earth as a whole is considered, clouds are
most important in determining albedo.
A LARGER AREA
Figure 1-6.Dispersion of insolation.