Cloud Seeding §Examples of man's desire to modify weather are as old as recorded history. § The modern era of weather modification began in the late 1940s when Irving Langmuir and his colleges at the G.E. Research Laboratory in New York dropped dry ice into a cold cloud layer, converting the water to ice, which led to the production of snow flakes and the dissipation of the cloud in the seeded region. § This experiment was followed by Vonnegut's discovery of the ice nucleating properties of silver iodide. § Seeding experiments and operational programmes using these two materials quickly spread all over the world. §Some experiments were conducted by the CSIR in South Africa in 1948.

Cloud Seeding §The basis for convective cloud seeding is centred upon the observation that the natural precipitation formation processes are frequently inefficient. § Ice crystals are added to clouds using the traditional glaciogenic materials, dry ice and silver iodide. Usually, silver iodide has been the preferred material because of its ease of dispersion (ground generators, flares attached to aircraft, droppable pyrotechnics and rockets).

Cloud Seeding - SA Research §Operational hail suppression programme was launched by the Lowveld Tobacco Co-operative in §After 10 years of seeding, this programme was terminated because of rising fuel and silver prices.

Cloud Seeding - SA Research §Two research programmes, one based at Bethlehem and the other at Nelpsruit, started rain augmentation experiments in the 1980s §Investigations showed that emissions from the a paper mill, were altering the cloud droplet spectra in clouds growing close to the mill in a manner that would increase or accelerate the coalescence formation of rainfall. §Thunderstorms in the summer rainfall regions of South Africa characteristically have strong updrafts. Precipitation particles forming in these updrafts may not grow large enough fast enough to escape the updraft. § Any process that can speed up the formation and growth of these particles should lead to the harvesting of more rainfall from such clouds.

Cloud Seeding - SA Research §Led to the development of a hygroscopic cloud seeding flare §New flares were mounted at the rear of the engine of seeding aircraft §research teams at Bethlehem and Nelspruit were amalgamated under a single banner - the National Precipitation Research Programme (NPRP). § Any process that can speed up the formation and growth of these particles should lead to the harvesting of more rainfall from such clouds. §to test the ability of the new cloud seeding technology to put more water on the ground to augment water resources. This involved the planning and execution of an area experiment.

Does it work? §60 seeded storms over a period produce65% more rain than the 60 unseeded storms that they have been matched with.

Does it work? - Streamflow studies §Studies in South Africa have indicated that a modest 7% increase in rainfall could produce a 25% increase in stream flows.