Environmental Engineering Reference
In-Depth Information
words, we can achieve rainfall generation only when the right type of clouds already
exist; we cannot produce clouds likely to give rain from a cloudless atmosphere.
It has been found that the most likely circumstances suitable for precipitation
formation, or cloud
seeding
, as it is known, is when there are plenty of water droplets
within the cloud, but there are insufficient ice nuclei to assist the enlargement of the
water droplets. By adding artificial ice nuclei, or by freezing existing water droplets
though adding frozen carbon dioxide (dry ice), under the right circumstances,
precipitation can be produced. Such clouds are most likely to exist over mountain ranges,
such as the Rockies, where moist air rising over the mountains produces the clouds, but
not necessarily precipitation. Experiments of adding dry ice to such clouds have led to an
increase in snowfall. Elsewhere results have been less clear. The complexity of
atmospheric processes in precipitation formation makes it difficult to produce statistically
convincing results of enhancement. Areas worst affected by drought rarely have the right
sort of clouds on which to conduct the experiments, so we are still a long way from being
able to produce precipitation at will.
Cloud seeding experiments in South Africa have added small salt particles (0·5 µm in
diameter) just below cloud base in an attempt to increase the cloud droplet sizes and
accelerate the coalescence process. The development of this approach was initiated by
radar and cloud observations near a large paper mill which indicated greater cloud
development near the chimney plume than elsewhere.
operate. Cloud thickness and updraught speed are largely dependent upon instability and
convergence in the atmosphere. Precipitation has been classified in terms of the factor
which gives rise to the upward movement, so let us have a look at this in a little more
detail.
TYPES OF PRECIPITATION
CONVECTIONAL PRECIPITATION
The spontaneous rising of moist air due to instability is known as convection. We have
seen that upward-growing clouds are associated with convection. Since the updraughts
are usually strong, cooling of the air is rapid and lots of water can be condensed quickly.
Collisions and coalescence are likely to be frequent, so the larger droplets rapidly
increase in size. Eventually, growing larger and heavier, the droplets overcome the lift
provided by the updraught, and they start to fall through the cloud into the clear air
beneath. As the volume of water in these big drops is large relative to their surface area,
little evaporation takes place in the non-saturated air below the cloud. At the ground there
will be a burst of heavy rain as the shower passes.
Unstable air which favours convectional rain is most frequently found in warm and
humid areas, but even in the United Kingdom about 20 per cent of the annual rainfall is
by convection, with the proportion increasing towards the south and east. This
convectional rain may be the result of cold air moving over a warmer ground surface or
the result of strong surface heating; both situations will give the steep lapse rates
characteristic of instability and convection.
