Geoscience Reference
In-Depth Information
The effect of cloud cover also operates in
reverse, since it serves to retain much of the heat
that would otherwise be lost from the earth by
longwave radiation throughout the day and night.
In this way, cloud cover appreciably lessens the
daily temperature range by preventing high
maxima by day and low minima by night. As well
as interfering with the transmission of radiation,
clouds act as temporary thermal reservoirs
because they absorb a certain proportion of the
energy they intercept. The modest effects of cloud
reflection and absorption of solar radiation are
illustrated in
figures 3.5 to 3.7
.
Global cloudiness is not yet accurately known.
Ground-based observations are mostly at land
stations and refer to a small (~ 250km
2
) area.
Satellite estimates are derived from the reflected
shortwave radiation and infrared irradiance
measurements, with various threshold assump-
tions for cloud presence/absence; typically they
refer to a grid area of 2500km
2
to 37,500km
2
.
Surface-based observations tend to be about 10
percent greater than satellite estimates due to
the observer's perspective. Average winter and
summer distributions of total cloud amount from
surface observations are shown in
Figure 3.8
. The
cloudiest areas are the Southern Ocean and the
mid- to high-latitude North Pacific and North
Atlantic storm tracks. Lowest amounts are over
the Saharan-Arabian desert area. Total global
cloud cover is just over 60 percent in January and
July. The low altitude cloud fraction is shown in
Plate 3.2
.
4 Effect of latitude
As
Figure 3.4
has already shown, different parts of
the earth's surface receive different amounts of
solar radiation. The time of the year is one factor
controlling this, more radiation being received in
summer than in winter owing to the higher
altitude of the sun and the longer days. Latitude
is a very important control because this deter-
mines the duration of daylight and the distance
travelled through the atmosphere by the oblique
rays of the sun. However, actual calculations show
the effect of the latter to be negligible near the
poles, apparently due to the low vapor content of
the air limiting tropospheric absorption.
Figure
3.7
shows that in the upper atmosphere over the
North Pole there is a marked maximum of solar
radiation at the June solstice, yet only about 30
percent is absorbed at the surface. This may be
compared with the global average of 48 percent of
solar radiation being absorbed at the surface. The
explanation lies in the high average cloudiness
over the Arctic in summer and also in the high
reflectivity of the snow and ice surfaces. This
example illustrates the complexity of the radiation
budget and the need to take into account the
interaction of several factors.
A special feature of the latitudinal receipt of
radiation is that the maximum temperatures
experienced at the earth's surface do not occur at
the equator, as one might expect, but at the
tropics. A number of factors need to be taken into
account. The apparent migration of the vertical
sun is relatively rapid during its passage over the
equator, but its rate slows down as it reaches
the tropics. Between 6°N and 6°S the sun's rays
remain almost vertically overhead for only 30 days
during each of the spring and autumn equinoxes,
allowing little time for any large buildup of surface
heat and high temperatures. On the other hand,
between 17.5
500
400
300
200
100
0
latitude the sun's rays
shine down almost vertically for 86 consecutive
days during the period of the solstice. This longer
interval, combined with the fact that the tropics
experience longer days than at the equator, makes
the maximum zones of heating occur nearer
°
and 23.5
°
90°S
60°S
30°S
0°
Latitude
30°N
60°N
90°N
Figure 3.7
The average receipt of solar radiation
with latitude at the top of the atmosphere and at
the earth's surface during the June solstice.
