The beginning of artificial rain-making goes back to 1946, just after World II, when V.J. Schaefer and E. Langmuir, two American scientists, discovered by their experiments in the General Electric Laboratory, New York that certain types of clouds may be modified to produce precipitation.
They dropped dry ice from an aeroplane into a super-cooled cloud over New England which in no time resulted in the growth of ice crystals. However, the falling drops of precipitation evaporated before reaching the ground.
Similar experiments in artificial rain making were made in Australia which resulted in partial success. But soon after the scientists were convinced that the super-cooled layers of clouds in general as well as the cumulus clouds in particular can easily be converted into ice-clouds.
No doubt, many a time the dry-ice technique succeeded in releasing precipitation from certain special types of clouds.
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Soon after the above discovery, B. Vonnegut (1949), another American scientist, found out another technique of artificial precipitation. His notable discovery was that at temperatures below -5° Celsius silver iodide could also be used for cloud seeding.
The fine powder of silver iodide acts as nuclei which produce ice crystals. Silver iodide smoke is introduced into suitable clouds by aeroplanes or by the use of explosive rockets or balloons. Even from a series of burners placed on the ground a continuous silver iodide smoke screen can be sent up to the clouds for their seeding.
The special characteristic of silver iodide is that when it is heated to a very high temperature, it converts into vapour. After cooling the vapour produces tiny crystals of 0.01 to 0.01 micron diameter. When such crystals are introduced into the super-cooled clouds, the entire cloud is immediately converted into ice-clouds.
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Acetone, a highly inflammable chemical is used for burning the silver iodide solution. However, it is to be noted that its use for artificial precipitation is effective only in such clouds as have potentialities for precipitation in natural way.
Another technique of inducing artificial precipitation is based on Bowen’s Coalescence Theory, which has already been discussed in this chapter. This process relies upon Ludlam’s (1951) Double Sweep Theory, and is applicable to warm clouds.
By this method water droplets of about 50 micron diameter are sprayed into the lower layers of deep clouds so that they may grow by sweeping up microscopic cloud particles during their upward and downward movements.
Still another technique for getting the release of precipitation from non-precipitating clouds is the injection of finely ground salt. These particles act as hygroscopic nuclei. Since these nuclei are always present in adequate number, this technique simply attempts to increase the proportion of large nuclei to stimulate raindrop production in a cloud.
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To summarize, the process of artificial rainmaking through cloud seeding consists of introducing artificial nucleation agents into super-cooled clouds.
Whatever the ultimate success with the above mentioned techniques, there are two fundamental considerations which must not be lost sight of:
(1) The clouds of great vertical extent must pre-exist before any rainmaking device is applied to them.
(2) According to Best and Mason, cloud seeding cannot unlock the reserves of water vapour in the atmosphere, since none of the seeding techniques contains any mechanism for renewing the cloud.
Most of the experts are of the opinion that the depth of clouds selected for seeding must exceed 1500 meters. Besides, the altitude of the bases of such clouds from the ground should be as much as their vertical thickness. Generally nimbus clouds, dark brown in appearance, are selected for artificial precipitation. Such clouds are found at a height of 1-2 kilometers from the ground.
Scientists are exploring the possibilities of generating artificial precipitation by means of lightning and shock waves. Convective activities in the atmosphere are affected by the atmospheric heat produced by lightning, and the electrical discharges help in the growth of cloud droplets by coalescence.
That is why there is torrential rainfall just after the flash of lightning in cumulo-nimbus clouds. In the same way, shock waves generated by thunder contribute a lot to the collision of cloud droplets which results in the formation of large-sized raindrops.
Because of these shock waves, the air expands and cools, so that the microscopic ice- crystals form in the cloud. These ice crystals generate precipitation. In the recent past, various attempts have been made on experimental basis to secure the release of precipitation by rocket explosion in the clouds.