References
183
Now, using Eq. (1 l.l4), with Fp = cos I) = cos 30 = 0.866 (the sun
zenith angle was given as 30"); Fd = Fa = 1, and F, = F, = 0,
gives Rabs = 0.74 x (0.866 x 938 w m-2 + 1 x 110 w m-2) + 0.97 x
(1 x 407 W m-2) = 1077 W m-2. The net radiation is ( E ~ .
(1 1.15)):
R,, = 1077 W m-2 - 496 W m-2 = 581 W m-2.
Example 11.4. If a sparrow were standing on the grass, what would its
Rabs be?
Solution. From Table 11.4, the sparrow absorptivity is 0.75. Consulting
Fig. 1 1.6 it can be seen that the value of Fp could range from about 0.15
to 0.35 depending on the orientation of the bird to the solar beam. We
calculate the absorbed radiation for both extreme values, using the values
from Example 1 1.3.
Smallest Rabs:
Largest Rabs:
Clearly, the bird has access to a wide range of absorbed radiation
environments, just by choosing its orientation with respect to the sun.
Example 11.5. If a single flat leaf were suspended horizontally over the
same grass surface, what would its absorbed radiation be?
Solution. From Table 11.4, the absorptivity for the leaf is around 0.5.
Since the leaf is horizontal, Fp = 0.5 x cos I) = 0.433. All of the other
view factors are 0.5, and F, = 1. The absorbed radiation is therefore:
References
Coulson, K. L. (1975) Solar and Terrestrial Radiation. Academic Press,
New York.
Gates, D. M. (1962) Energy Exchange in the Biosphere. New York:
Harper and Row.
Gates, D. M. (1965) Radiant energy, its receipt and disposal. Meteor.
Monogr, 6: 1-26.