There have been several attempts to force clouds to unload their moisture onto parched lands. But these were unsuccessful until 1946, when Vincent Schaefer in the United States first demonstrated that artificially introducing nuclei into a cloud could stimulate rainfall. Initial experiments were encouraging and prompted research in several countries, including Australia (Note 9.E).
In the case of warm clouds, raindrop formation can be stimulated by introducing water-absorbing particles of sea-salt or ammonium nitrate with urea. The material is first ground to particles of about 3 pm. The resultant large droplets promote the accretion of raindrops.
Most success in cloud seeding has been achieved with convective cold clouds. Glaciation of the supercooled droplets can be achieved with particles of either 'dry ice' or silver iodide (Note 9.E). The most common method nowadays (in Australia, the USA and Israel) involves releasing silver iodide from burners on an airplane, flying either in the updraught below cumulus clouds or within stratus near the -10°C level. Ice crystals form in a suitable cloud within a few minutes, and rain may fall about twenty minutes later. The effect on convective clouds appears short-lived, but additional stratiform precipitation may continue for over an hour after seeding.
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