Rain making, or Cloud-Seeding has been around since 1946, when Dr. Bernard Vonnegut, the brother of  author Kurt Vonnegut and an MIT-trained Chemist, discovered while employed at General Electric that silver-iodide had a similar crystal lattice-structure to ice and suggested it might be a particularly good chemical to use as a nucleation-promotor for super-cooled moisture the atmosphere.  In other words, if there's enough moisture, and it's cold enough, but conditions aren't quite right for rain production - "seeding" clouds with silver iodide, solid carbon-dioxide "dry-ice" or certain salts can make rain slightly more probably.

Rain making is actually part of the larger field of weather-modification, and much more common and widespread than most people think - believing it to still be in the realm of Science Fiction.  During the VietNam war, the US Military executed Operation Popeye, which extended the monsoon season over Ho-Chi-Min City.  Todsy, China maintains the world's largest weather-modification effort, mostly aimed at promoting rain in the arid regions surrounding Beijing. which has caused some internal political unrest because downwind cities of less influence claim that what would normally be their rain is regularly stolen.  Water purity and availability will continue to be one of the major challenges facing China's expansion efforts, and represents significant opportunities for those able to remedy an already tense situation.

From Asia Times:

... The farmers are part of the biggest rain-making force in the world: China's Weather Modification Program.

According to Wang Guanghe, director of the Weather Modification Department under the Chinese Academy of Meteorological Sciences, each of China's more than 30 provinces and province-level municipalities today boast a weather-modification base, employing more than 32,000 people, 7,100 anti-aircraft guns, 4,991 special rocket launchers and 30-odd aircraft across the country.

... In the beginning, the idea was to ease drought and improve harvests for Chinese farmers, but over the decades other functions have evolved such as firefighting, prevention of hailstorms, and replenishment of river heads and reservoirs. Artificial rain has also been used by some provinces to combat drought and sandstorms. In 2004, Shanghai decided to induce rain simply to lower the temperature during a prolonged heat wave to bring relief to an increasingly hot and sweaty urban populace.

... Zhang Qiang, the top weather-modification bureaucrat in Beijing, said her office has been conducting experiments in cloud-busting for the past two years in preparation for the Games' opening ceremony on August 8, 2008.

She said that according to past meteorological data, there is a 50% chance of drizzle on that day. To ensure blue skies, the Beijing Weather Modification Office is busy researching the effects of various chemical activators on different sizes of cloud formations at different altitudes. The aim is to catch pregnant clouds early and induce rainfall ahead of the big day so that during the opening ceremony the sky is cloud-free.

While declining to provide specifics, Zhang said her office's budget has seen sharp spikes in recent years and she expects it to continue to grow given northern China's extreme water shortages, which are exacerbated by the impact of climate change. Indeed, the annual per capita water supply for China is only 2,200 cubic meters, just 25% of the global average, according to the World Bank.