Geoscience Reference
In-Depth Information
Figure 3.3
Perihelion shifts. (A) The present timing of perihelion. (B) The direction of its shift and the situation at 11,000 years
BP
.
(C) The geometry of the present seasons (northern hemisphere).
Source
: Partly after Strahler (1965).
4 per cent more radiation than today (Figure 3.3B).
This same pattern will return about 10,000 years from
now.
Figure 3.4 graphically illustrates the seasonal varia-
tions of energy receipt with latitude. Actual amounts
of radiation received on a horizontal surface outside
the atmosphere are given in Table 3.1. The intensity on
a horizontal surface (
I
h
) is determined from:
3 Altitude of the sun
The altitude of the sun (i.e. the angle between its rays
and a tangent to the earth's surface at the point of
observation) also affects the amount of solar radiation
received at the surface of the earth. The greater the sun's
altitude, the more concentrated is the radiation intensity
per unit area at the earth's surface and the shorter is the
path length of the beam through the atmosphere, which
decreases the atmospheric absorption. There are, in
addition, important variations with solar altitude of the
proportion of radiation reflected by the surface,
particularly in the case of a water surface (see B.5, this
I
h
=
I
0
sin
d
where
I
0
= the solar constant and
d
= the angle between
the surface and the solar beam.
