Chinese emperors were once thought by their subjects able to make it rain with their prayers. Chinese officials today put more trust in science. They are boosting spending on what they call the world's largest programme of artificial rainmaking. Its effects, however, are not much easier to quantify. 

Just as grateful subjects praised the heavens for any rain that fell, the state-controlled media in China often report on the heroic contributions made by "weather-modification offices" whenever needed rain comes. These offices, set up over the past 15 years, deploy artillery（火炮）, rocket-launchers and aeroplanes to seed clouds with chemicals （usually silver iodide） that encourage droplets to form and fall where needed, or prevent the formation of destructive hailstones.

China's efforts have been encouraged by an increasingly desperate shortage of water in the north. China also frets about the Olympic games in Beijing in August 2008, in a season when rainstorms are common. Making it rain away from big events is a key area of research. Last week officials awarded merit certificates to organisations involved in "weather guarantee" work for a summit with African leaders held in Beijing in November. Those honoured included the air force, Beijing's military command and the Second Artillery, which controls missiles. All were urged to do a good job at an Olympic rehearsal next year.

Artificial rainmaking is practised in many countries but its success is hard to prove. In recent years, China has been helped by big improvements in its weather-monitoring capacity. Its latest satellite, Fengyun-2D, was launched on December 8th with Olympic forecasting as a priority mission. It plans to launch another 22 weather satellites by 2020.

Mao Jietai of Peking University says circumstantial evidence suggests cloud seeding can increase precipitation by around 10%, but admits doubts about its real efficacy. Undaunted, the government has set an annual target for artificially induced rain nationwide for the next five years: around 50 billion cubic metres, or five times parched Beijing's average. As in imperial times, few will be able to refute claims of success.