Environmental Engineering Reference
In-Depth Information
After the snowflakes have melted the resulting drops may grow further by collision with
cloud droplets before they reach the ground as rain. This method of producing raindrops
is known as the Bergeron-Findeisen process, after the developers of the theory (Figure
4.6). Beneath the base of the cloud, however, evaporation will take place in the drier air
and if the drop is small it may be evaporated completely.
Precipitation formation both by collision and coalescence and by the Bergeron-
Findeisen process undoubtedly occurs in the atmosphere, though clearly the Bergeron-
Findeisen process can operate only when cloud temperatures are well below freezing.
The rate at which vapour is converted into water droplets and precipitation depends upon
three main factors: the rate of coalescence and ice crystal growth; the cloud thickness;
and the strength of the updraughts in the cloud. The total amount of rain will be
determined by the life span of the cloud, the height of the cloud above the ground and
how long these processes
Figure 4.6
Schematic diagram to demonstrate the processes
of precipitation growth in (left) stratiform and (right)
cumuliform clouds.
CLOUD SEEDING
new developments
In many parts of the world either the total amount or the irregular distribution of rainfall
means that serious deficiencies can occur leading to water shortages. It has long been the
desire of farmers to be able to persuade clouds to produce precipitation at their command
rather than waiting for natural precipitation to fall. Unfortunately experiments have not
been particularly successful. For the realistic production of precipitation, the natural
processes have to be helped when there are factors limiting their operation
In other
