Geoscience Reference
In-Depth Information
Figure 4.25 Average seasonal changes in
net radiation on Earth.
(a) Mean January
net radiation. (b) Mean July net radiation. In
both figures, reds indicate high values of net
radiation, measured in watts per square meter,
whereas blues equal low or deficit amounts.
(a)
(b)
Added insolation
at pole
90° N
80° N
70° N
Equator and the North Pole, 45° N has a moderate range of
daily insolation, with a peak during the summer months and
lower values during winter.
In addition to these patterns, Figure 4.27 reveals two
other interesting relationships. The first is that the Equator
shows two peaks in daily insolation, rather than one, over the
course of the year. The reason for these dual peaks is that the
Sun is directly over the Equator two times during the year—
specifically, at each Equinox. The second interesting pattern
is that the North Pole (90° N) receives more insolation around
the Summer Solstice than the Equator, even though the angle
of incidence at very high latitudes is still relatively low. Why
does this happen? The answer is that latitudes above the Arctic
Circle receive radiation for 24 h because the Sun never sets. At
the Equator, in contrast, day length is always about 12 h. This
pattern shows the significance of day length for the amount of
daily insolation received, which, in turn, depends on axial tilt
and orbital position.
Insolation with
Earth axis tilted
60° N
50° N
40° N
30° N
20° N
10° N
0°
Insolation if Earth
axis were upright
Decreased insolation at
equatorial/tropical regions
due to axial tilt
10° S
20° S
30° S
40° S
50° S
60° S
70° S
Added insolation
at pole
80° S
Figure 4.26 Variation in annual insolation (by latitude)
on Earth.
The red line shows the variability that exists
as a function of the actual axial tilt. The blue line indicates
what insolation would be if the axis were not tilted. Note the
added insolation at high latitudes due to the tilt of Earth.
90° S
0
100
200
300
400
500
Annual insolation, W/m
2
