From: North American Weather Consultants, Inc.
http://www.nawcinc.com/wmfaq.html"---Cloud Seeding Frequently Asked Questions---"
"Is cloud seeding effective?
...For precipitation augmentation, the accepted magnitude of increase to be expected from well-designed and properly conducted projects ranges from 5% to 20% for winter precipitation in continental regions and from 5% to 30% for coastal areas. For warm season precipitation increase, single-cloud experiments have indicated increases as large as 100%. Area wide increases over a project area vary with the frequency of occurrence and spatial coverage of suitable cloud systems, plus the ability to treat all favorable clouds. Hail suppression effectiveness, based upon surface hail data, is estimated to be in the range of a 20-50% reduction."
"How widely used/accepted is cloud seeding technology?
Over its history of 50 years, modern cloud seeding has involved projects of various types in nearly 50 countries around the world. Some individual projects have been in operation nearly continuously for decades, with a few operating for nearly fifty years. As water needs increase world-wide, the demand for weather modification services will also increase. Similar increase in demand will occur in hail-prone regions."
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From: BBC News, Thursday, 17 March 2005
http://news.bbc.co.uk/2/hi/asia-pacific/4358535.stm"Thailand's King Bhumibol Adulyadej is to oversee personally an artificial rain-making project in a bid to end a severe drought in the country..."
It's been used in Thailand this year. I'm told that it worked, but I don't know how well. From what I've read, the chemical used was probably Silver Iodide.