Geoscience Reference
In-Depth Information
typically they refer to a grid area of 2500 km
2
to 37,500
km
2
. Surface-based observations tend to be about 10 per
cent 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 (see
Plate 1). Total global cloud cover is just over 60 per cent
in January and July.
4 Effect of latitude
Figure 3.6
Percentage of reflection, absorption and transmission
of solar radiation by cloud layers of different thickness.
Source
: From Hewson and Longley (1944). Reprinted with permis-
sion. Copyright © CRC Press, Boca Raton, Florida.
As Figure 3.4 has already shown, different parts of
the earth's surface receive different amounts of solar
radiation. The time of year is one factor controlling this,
more radiation being received in summer than in winter
because of the higher altitude of the sun and the longer
days. Latitude is a very important control because this
determines 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, due appar-
ently to the low vapour 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 per cent is absorbed at the surface. This may
be compared with the global average of 48 per cent
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 thirty 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°
and 23.5° latitude the sun's rays shine down almost
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.
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 long-wave radiation throughout
the day and night. In this way, cloud cover lessens
appreciably 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 known accurately.
Ground-based observations are mostly at land stations
and refer to a small (~ 250 km
2
) area. Satellite estimates
are derived from the reflected short-wave radiation
and infra-red irradiance measurements, with various
threshold assumptions for cloud presence/absence;
