Environmental Engineering Reference
In-Depth Information
Figure 3.13
The effect of water on diurnal temperature
ranges. Oceanic sites have a small diurnal temperature
range, whereas in a desert the range is high.
so temperatures are high, while radiational cooling at night is intense, giving rise at times
to low air temperatures.
THE SEASONAL PATTERN
A very similar pattern of variation takes place on a seasonal scale. The cause in this case
is not Earth's rotation but its changing relationship with the sun: the variation within its
orbit that produces the apparent seasonal progress of the sun from the Tropic of Cancer to
the Tropic of Capricorn and back.
This change in the position of the sun leads to changes in the angle of the incoming
rays and in the duration of daylight. Both factors influence the amount of insolation
received by Earth and, therefore, the degree of atmospheric heating. Considering again
our area in London, we would find that in the winter the maximum elevation of the sun,
at midday, was about 16°, for the sun stands approximately over the Tropic of Capricorn.
Thus the rays of the sun still strike the surface at a relatively low angle and the degree of
midday heating is limited.
As the sun moves northwards to the equator and thence to the Tropic of Cancer its
midday position rises and the rays strike the surface less obliquely. Moreover, the days
become longer and the nights shorter. Maximum temperatures increase until, about July,
they reach their highest values, slightly after the maximum radiation in late June. From
then until mid-December the sun returns south, its midday position in the sky declines,
the quantity of insolation received at the surface is reduced and so temperatures fall.
