Page 292 - Fundamentals of Air Pollution

248 17. The Physics of the Atmosphere

Fig. 17-2. Orientation of the earth to the solar beam at the extremes of its revolution
around the sun.

ern Hemisphere), 8 is solar declination (see Table 17-2), and 77 is hour angle,
15° x the number of hours before or after local noon.
The solar azimuth &> is the angle between south and the direction toward
the sun in a horizontal plane:

Since many surfaces receiving sunlight are not horizontal, a slope at
an angle i from the horizontal facing an azimuth o>' degrees from south
experiences an intensity of sunlight (neglecting the effects of the atmo-
sphere) of

Here to and co' are negative to the east of south and positive to the west.
At angles away from the zenith, solar radiation must penetrate a greater
thickness of the atmosphere. Consequently, it can encounter more scatter-
ing due to the presence of particles and greater absorption due to this
greater thickness.

TABLE 17-2
Solar Declination"
Declination Declination
Date degree Date degree
Jan. 21 -20.90 Jul.21 20.50
Feb. 21 - 10.83 Aug. 21 12.38
Mar. 21 0. Sep. 21 1.02
Apr. 21 11.58 Oct. 21 -10.42
May 21 20.03 Nov. 21 -19.75
June 21 23.45 Dec. 21 -23.43
" Adapted from Table 2-1 of Byers (2).