Environmental Engineering Reference
In-Depth Information
Figure 3.12
Diurnal changes in short-wave radiation input
and temperature on a clear day.
sun rises, the ground warms up and, in turn, the air in contact with the surface is heating
too (Figure 3.12). Between about 2.00 and 3.00 p.m. the ground and air are usually at
their warmest, the maximum temperature at the surface being earlier than that in the air
because that is where the heat conversion takes place. From then on, as the sun gradually
sinks, the ground surface and the overlying air will cool. The sun sets at about 6.00 p.m.;
cooling continues throughout the night until minimum temperatures are reached just
before dawn.
This daily variation in insolation and temperature is one of the most basic components
of our climate. So obvious is it and so regular that we take it for granted. And yet quite
marked differences in atmospheric conditions occur in response to the daily progress of
the sun. As we shall see later, the associated changes in temperature may lead to
significant changes in humidity, and they often spark off major atmospheric processes
such as vertical movements of air and even heavy storms.
It is also apparent that this daily pattern of insolation and temperature change itself
varies according to atmospheric conditions. The effects are most obvious when the air is
clear and still, for then heating and cooling proceed uninterrupted. If the sky is cloudy or
very hazy, however, the daily pattern of temperature is much more variable (Figure 3.4).
Similarly, the pattern varies spatially. It is less marked over the sea, for much more of the
incoming energy is used to heat up and evaporate the water, and less is returned directly
to heat the atmosphere. During the night the sea cools slowly, with the result that
temperatures do not fall so much as on land - one reason why coastal areas are less prone
to night-time frost (Figure 3.13). The pattern is most apparent in areas with dry air and
ground surfaces, such as deserts. There, incoming radiation is large, and little energy is
used for evaporation,
