Atmosphere does not convert gaseous vapour into liquid water in a very efficient manner. Substantial material benefits could be derived if the efficiency could be enhanced. With this objective in mind, a number of experiments have been designed in the last two decades to induce clouds to release precipitation. Most experimenters have worked on the hypothesis that, if the number of natural nuclei in a cloud could be increased, this, in turn, would augment the rate of drop formation and, ultimately, a large quantum of water would be released from the cloud.
A few decades back, Vincent Schaefer, an American scientist, discovered that a tiny piece of dry ice, when dropped into a cold chamber filled with super-cooled cloud, resulted in the formation of a very large number of ice crystals. Schaefer was closely associated in this work with Irving Langmuir, who had earlier won a Nobel Prize for his research in physical chemistry, and they estimated that a pellet of dry ice, about the size of a pea, could produce about 10 to the power of 16 ice crystals in its passage through a super cooled cloud.
The first field trials on cloud seeding appear to have been conducted by Schaefer and Langmuir towards the end of 1946. Observations from the ground suggested that snow fell from the seeded cloud for about 0.6 km before evaporating in the dry air. At about the same time, experiments by Kraus and Squires in Australia reported that six out of eight seeded clouds gave radar echoes after seeding, and in four cases rainfall was reported at the ground.
In later years Vonnegut, another associate of Professor Langmuir, found that silver iodide was a more powerful agent than dry ice for producing ice nuclei. Vapourising an acetone solution of silver iodide by a burner could produce enormous numbers of nuclei. There was the possibility that a large number of nuclei could be disbursed from ground-based burners, and a few experiments in this direction appear to have indicated encouraging results.
For warm clouds, whose tops do not extend to the freezing level, attempts have been made to release showers by spraying Cumulus clouds with water or aqueas salt solutions. The objective is to introduce large drops of water or a suitable chemical at the base of the cloud to initiate and expedite the coalescence mechanism.
Acknowledgement : Excerpt from ‘The Monsoon’ by Dr.P.K.Das, former Director General of the National Meteorological Service of the Government of India. Published by National Book Trust, India. Price: Rs.75/=