Geoscience Reference
In-Depth Information
Figure 3.8
The global distribution
of total cloud amount (per cent)
derived from surface-based obser-
vations during the period 1971 to
1981, averaged for the months June
to August (above) and December to
February (below). High percentages
are shaded and low percentages stip-
pled.
Source
: From London
et al
. (1989).
vertically for eighty-six 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 the tropics than the equator. In the northern
hemisphere, this poleward displacement of the zone of
maximum heating is enhanced by the effect of
con-
tinentality
(see B.5, this chapter), while low cloudiness
associated with the subtropical high-pressure belts is an
additional factor. The clear skies allow large annual
receipts of solar radiation in these areas. The net result
of these influences is shown in Figure 3.9 in terms of
the average annual solar radiation on a horizontal
surface at ground level, and by Figure 3.10 in terms of
the average daily maximum shade temperatures. Over
land, the highest values occur at about 23°N and
10-15°S. Hence the mean annual
thermal equator
(i.e.
the zone of maximum temperature) is located at about
5°N. Nevertheless, the mean air temperatures, reduced
to mean sea-level, are related very broadly to latitude
(see Figures 3.11A and B).
5 Effect of land and sea
Another important control on the effect of incoming
solar radiation stems from the different ways in which
land and sea are able to profit from it. Whereas water has
a tendency to store the heat it receives, land, in contrast,
quickly returns it to the atmosphere. There are several
reasons for this.
