Environmental Engineering Reference
In-Depth Information
Figure 2
Atmospheric factors affecting the form of cumuliform
clouds.
radiational cooling, air movement and condensation. It forms only when cooling occurs
faster than the rate at which latent heat is added by condensation. Because vapour is
converted into water droplets, the moisture content and saturation temperature fall, so
further cooling is necessary to give saturation. Because of this, fog is more frequent
during the long nights of autumn, winter and spring than in summer. If winds are strong,
the saturated air near the ground will mix with drier air above and prevent fog forming.
By reducing visibility, fog can be a major environmental hazard. Fortunately reduction
in levels of smoke mean that dense fogs (smogs) are much less frequent than they used to
be.
CLOUDS
Clouds and fog are the result of similar processes which vary in intensity and duration.
Clouds are composed of a mass of water droplets or ice crystals almost microscopic in
size. The number of droplets per unit volume of cloud varies considerably, depending
upon its origins; smaller concentrations of larger droplets occur in clouds formed in the
middle of large oceans, while large concentrations of smaller droplets are found in
continental regions. Clearly this is a consequence of the greater availability of nuclei over
the dusty continental interiors, but polluted industrial areas may have a similar effect.
Studies of such condensation nuclei have shown that there are two broad classes: those
with an affinity for water, called hygroscopic particles, like salt; and non-hygroscopic
particles, which require relative humidities above 100 per cent before they can act as
centres of condensation. The role of sea salt as a source of hygroscopic nuclei has long
been debated but recent work suggests that it is not as important as was once thought.
We can find out much about what is happening in the atmosphere by looking at the
type of clouds and especially their shape (see box). The low- and medium-level stratiform
types are the main rain-bearing clouds of temperate latitudes often associated with
depressions. Around their centre we often see a characteristic sequence of clouds as the
warm air associated with the depression approaches (Figure 7.6).
