4. 6 Radiation balance
Radiation is emitted from a body whose temperature is above 0 K; in the present
situation, that is. everything emits radiation. Radiation is always considered a balance. For
the radiation balance of the bare ground, net radiation is defined as the difference between
incoming radiation from the sky and the outgoing radiation from the ground. In the
daytime, the balance includes solar radiation, of course. The net radiation only for
long-wave radiation is called effective radiation. In the nighttime, net radiation is equal to
effective radiation.
Radiation is like light; it can be considered to travel in a straight line. If two bodies,
such as the sky and the ground, are placed face to face without any intermediary, the
radiation exchange between these two bodies is one to one. The heat balance systems we
have so far considered are of this type - that is, the bodies involved are the flat soil surface
and the sky. These two surfaces can be considered infinite and parallel. Therefore, the
radiation emitted from one surface always enters the other.
But when another body such as a building is involved, as shown in Fig. 4.12, part of the
radiation from the ground cannot go directly to the sky, and vice versa. This situation can
be visualized by looking at the figure from the direction shown by the word "eye". Suppose
the building is infinite in the direction perpendicular to the page: then half of the sky and
half of your perspective is occupied by the face of the building. The ground does receive
half of the radiation from the sky, but it also receives half of that from the building surface.
In this case, it is said that the view factor of the ground to the sky is 0.5 and the view factor
of the ground to the building is 0.5. On the other hand, if you move your eye from the
ground to the building surface, it is clear that the view factor of the building to the sky is 0.5.
Of the 3 items being discussed, normally the sky has the lowest temperature and the smallest
amount of long-wave radiation. On calm clear nights, the soil surface is cooled by radiation