CLOUD seeding as a method of making it rain was discovered by Vincent Schaefer, a scientist with General Electric in New York.
Following observations he made while climbing Mount Washington, Mr Schaefer noted that, when frozen carbon dioxide was sprayed into a cold cloud chamber, ice began to form.
His experiments led him to depositing 1.5kg of dry ice, and later the li
ghter substance silver iodide, into clouds over Massachusetts via an aircraft. Despite his inconclusive results, the process became recognised, and is now known as glaciogenic seeding – dealing with, as it did, cold stratocumulus clouds.
Later, in 1989, warm cloud seeding was begun by a team of South African scientists who observed a cumulus cloud located above a paper mill producing large raindrops.
They reasoned that the drops were caused by the emissions from the mill's chimney and later mimicked its output by depositing salt crystals into other warm clouds.
The results were encouraging and in 1997 the South African government funded a much larger programme of warm cloud seeding above the extremely dry Limpopo province.
Before that, cold cloud seeding had been used by the US military in Vietnam in Operation Popeye.
From 1967 to 1972, clouds were seeded above the Ho Chi Minh Trail in an attempt to extend the monsoon season and increase the amount of mud the Viet Cong had to deal with.
In 1969, conspiracy theorists at Woodstock claim to have witnessed military planes seeding the clouds above the festival site, something they claim caused the heavy rain experienced during the event.
Cloud seeding is currently used widely in China. There, the government has allocated vast budgets in an attempt to modify the weather.
This saw the tactic widely deployed in the run up to the Beijing Olympics to ensure events were not disrupted by rain.
As well as light aircraft and artillery, the chemicals involved in cloud seeding can be dispersed into the air using ground-based generators that use warm, upward air currents to send the substances into the clouds.
