The Coalescence’ process of rain making was discovered by E. G. Boven of Australia. This process is applicable only to those clouds which do not extend beyond the freezing level.
Until 1940s it was the common belief that all precipitation originated through the Bergeron process. Only light drizzle was supposed to come from clouds located well below the freezing point.
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If it were true, then substantial precipitation could only be generated from such clouds as extended to such heights where temperatures would be much below the freezing point. But this is not true. In the tropics, cumulus clouds with only 2400 m thickness start giving precipitation over the oceans.
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The temperature in the uppermost part of these clouds seldom falls below 7°C. It is, therefore, true that ice particles contribute little in the development of rain drops of sufficient size in such warm clouds.
The occurrence of precipitation from such clouds involves the coalesces of cloud droplets of different sizes. Since the rate of fall of these unequal particles is different, they collide with each other within the cloud, and the larger drops grow at the expense of the smaller ones.
In fact, the rate of growth of falling water droplets depends on variables like the size and size distribution of the drops, and their concentration in the cloud.
Clouds made up exclusively of liquid droplets can generate precipitation only when they contain droplets larger than 20 micrometers. For the formation of these large droplets ‘giant’ condensation nuclei are required.
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These nuclei are provided by such hygroscopic materials as sea salt. Condensation on these hygroscopic nuclei starts even before the air reaches saturation point (relative humidity 100 per cent). Drops that have grown on such large condensation nuclei become relatively larger.
Since the rate of fall is size-dependent, the larger drops fall faster (or in a rising cloud, ascend more slowly) than the smaller ones. As such, they collide with the smaller droplets and coalesce.
In clouds with great vertical thickness and abundant moisture, cloud droplets are repeatedly carried upward and downward by ascending and descending air currents. Hence these drops quickly reach the required size.
According to Riehl, the following conditions are helpful for the growth of droplets to the required raindrop size: (a) Initially few condensation nuclei should be present so that there are not too many competing for the available water vapour, (b) Size of the nuclei should cover a good range so that different fall velocities will develop.
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If a cloud contains only very small droplets of uniform size, there will be colloidal stability in it and it cannot yield precipitation. This is so because in such a case all the droplets will descend slowly at a uniform rate.
Hence there shall exist little chance of collision among the drops. Besides, for the development of a single rain drop, it will take much time for about 1 million cloud droplets to coalesce.
As regards the required size of rain drops, it is to be noted that Water drops must have a diameter of more than 100 microns even in humid air. According to Riehl, a drop of 500 micron diameter would hardly take 10 minutes to reach the ground from a cloud base 100 m above the earth’s surface.
But a drop with a diameter of only 10 microns would take at least 50 hours to cover the same distance. A radius of 500 microns has been adopted as the minimum size of a rain drop. Below this, rain drops are referred to as ‘drizzle’.
Most of the cloud droplets are so small that the motion of air keeps them suspended. Even if these microscopic droplets were to fall, they would evaporate before reaching the surface.
It has been demonstrated by high-speed motion pictures that sometimes mere collisions do not lead to coalescence. Besides collision, electrical attraction between the droplets plays an important role in bringing about coalescence.
If the colliding droplets have opposite electrical charges, coalescence is easily achieved. The role of electrical attraction in generating precipitation from a cloud has been investigated by Bernard Vonnegut and Charles B. Moore.
Whatever process or processes are at work, a continual supply of moisture to the atmosphere over a particular location is the most important condition for ensuring a prolonged precipitation.