Radiation Balance
175
+no
cloud
+cirrus
+cirrostratus
t altocumulus
+altostratus
+stratocumulus
*stratus
-+-nimbostratus
0
10
20 30 40 50
60
70
80
90
Solar Elevation Angle (degrees)
FIGURE 1 1.4. Solar irradiance under cloud cover for various cloud types and solar
elevation angles (data from List, 197 1).
Example 11.2. Find all components of the solar radiation over a grass
surface on a clear day when the solar zenith angle is 30" and the altitude
is 800 m.
Solution. The cosine of the zenith angle is needed in several places; it
is cos(30) = 0.866. The atmospheric pressure is (Eq. (3.7)).
p, = 101 exp - = 91.6 kPa.
( z: )
The airmass number is (Eq. 1 1.12)
The transmittance, t m is needed to compute both Sp and Sd. Assuming
a value of 0.7 for t gives 0.7l.O' = 0.69. Now, using Eq. (1 1.11), S, =
1360 W m-2 x 0.69 = 938 W mV2. Using Eq. (11.13), Sd = 0.3 x
1360 W m-2 x 0.866 x (1 - 0.69) = 110 W m-' . Using Eqs. 11.8
and 1 1.9, St = 938 W m-2 x 0.866 + 110 = 922 W m-2. Finally, the
reflected radiation is obtained from Eq. (1 1.10) using the average grass
albedo from Table 11.2: S, = 0.26 x 922 W m-2 = 240 W m-2.
11.4 Radiation Balance
Part of the radiation incident on a leaf, an animal, a crop canopy, or a
soil surface is absorbed at the surface. This energy can warm the surface