Geoscience Reference
In-Depth Information
Observer sees
white Sun, blue
sky at noon
White light
from Sun
North Pole
Direction of
rotation
Observer sees
red sunset
White light
from Sun
Blue light
scattered
(b)
(a)
Figure 4.18 Colors in the sky. (a) The sky is blue because oxy-
gen and nitrogen molecules scatter light in the blue wavelength.
(b) Geometric relationship of insolation, atmospheric thickness,
and sky color. (c) Colors at sunset along the Atlantic coast at
Rhode Island. Oranges and reds appear because the blue wave-
length is completely scattered before it reaches your eye.
Some incoming solar radiation in the atmosphere is also
redirected by relection . The amount of radiation reflected de-
pends on the albedo of a surface and takes place in the atmo-
sphere when insolation bounces off bright cloud tops. (Albedo
refers to the amount of reflection a given surface can cause; a
high albedo is typical of bright surfaces such as a mirror. We
will come back to this factor shortly.) Approximately 21% of
radiation is reflected back to space in this manner, and clouds
are by far the most important reflector.
In addition to the amount of radiation reflected, an addi-
tional component of incoming radiation is redirected within the
atmosphere by a process called scattering . Although scattering
cannot be directly observed, you can see the indirect effects very
clearly in the apparent colors of the sky (Figure 4.18). Have you
ever wondered why the sky is blue? The reason is that oxygen
and nitrogen molecules cause solar radiation to bounce around
in the air in a process called Rayleigh scattering . This effect is
more pronounced during the middle part of the day when the
Sun is high in the sky and solar radiation is streaming more or
less directly toward you. The sky appears to be blue because the
(c)
Reflection The process through which solar radiation is re-
turned directly to space without being absorbed by Earth.
Scattering The redirection and deflection of solar radiation
by atmospheric gases or particulates.
Albedo The reflectivity of features on the Earth's surface or
in the atmosphere.
 
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