Using a process called cloud seeding, they fire cigarette-sized pieces of silver iodide into the sky. The silver iodide in the sky causes moisture to collect and form around them in turn creating rain clouds. Eventually the moisture becomes heavy enough for it to fall as rain.
This department of weather gods employs over 37,000 workers, trained to use 37mm anti-aircraft guns, rocket launchers, and aircraft needed for all your weather-controlling purposes. Between 1995 and 2003, China's overall precipitation increased by more than 7.4 trillion cubic feet. Hurray for chemical rain!
Cloud seeding can be done by ground generators, plane or rocket (not shown).