Page 291 - Fundamentals of Air Pollution

I. Sun, Atmosphere System, and Heat Balance 247

is around 0.5-0.8, that of fields and forests is 0.03-0.3, and that of water
is 0.02-0.05 except when the angle of incidence becomes nearly parallel to
the water surface. Table 17-1 shows the albedo of a water surface as a
function of the angle of incidence. The albedo averaged over the earth's
surface is about 0.35.
Although events taking place on the sun, such as sun spots and solar
flares, alter the amount of radiation, the alteration is almost entirely in
the x-ray and ultraviolet regions and does not affect the amount in the
wavelengths reaching the earth's surface. Therefore, the amount of radia-
tion from the sun that can penetrate to the earth's surface is remarkably
constant.
In addition to the effect of albedo on the amount of radiation that reaches
the earth's surface, the angle of incidence of the radiation compared to the
perpendicular to the surface affects the amount of radiation flux on an area.
The flux on a horizontal surface S h is as follows:

where S is the flux through an area normal to the solar beam and Z is the
zenith angle (between the local vertical, the zenith, and the solar beam).
Because of the tilt of the earth's axis by 23.5° with respect to the plane
of the earth's revolution around the sun, the north pole is tilted toward
the sun on June 22 and away from the sun on December 21 (Fig. 17- 2).
This tilt causes the solar beam to have perpendicular incidence at different
latitudes depending on the date. The zenith angle Z is determined from:

where $ is latitude (positive for Northern Hemisphere, negative for South-

TABLE 17-1
Percent of Incident Radiation Reflected by a
Water Surface (Albedo of Water) 0

Angle of Percent Percent
incidence reflected absorbed
90 2.0 98.0
70 2.1 97.9
50 2.5 97.5
40 3.4 96.6
30 6. 94.
20 13. 87.
10 35. 65.
5 58. 42.
" Adapted from Fig. 3-13 of Battan (1).