They have been making rain in China recently. "One day last month," wrote Miriam Donohoe in last Tuesday's Irish Times , "the skies over Beijing turned black and it lashed torrential rain for several hours.
"The sudden downpour. . . was not an act of God, but the result of an army of 100 part-time rainmakers shooting rockets loaded with silver iodide into the clouds over the Chinese capital to create artificial rain."
"Cloud-seeding" to encourage rain originated in 1946 when an American chemist, Vincent Schaefer, found that if powdered carbon dioxide, or "dry ice", were dropped into a bank of cloud from an aircraft, it sometimes resulted in a fall of rain.
As we have seen in the China report, the particles may also be injected from below, by rocket. In either case, the particles of dry ice, or the silver iodide, act as a catalyst, encouraging the water droplets of the cloud to change into natural ice crystals, which enhances a cloud's ability to produce raindrops.
In the 1950s and 1960s in the US it became a fad. The thirsty farmers and ranchers of the Great Plains enlisted the aid of crop-dusters to "seed" the summer cumulus clouds. There was a spate of popular magazine articles on these glamorous efforts, but meteorologists were not so sure it worked.
One can't manufacture rain from nothing. The best one can hope for is to give nature a slight push, to add an ingredient to initiate a process on the point of happening anyway, but which might not have occurred without persuasion.
It is difficult to find a measure of success; rain may well occur after the operation, as obviously happened in Beijing, but who is to say it might not have rained anyway without human intervention?
During the 1980s, detailed studies were carried out in Australia, Israel and the US, and the report declared: "In the case of the Tasmania and Florida experiments the evidence does not provide strong support for a positive seeding effect.
"Israeli statistical evaluation provides more convincing evidence, and suggests an average increase in rainfall due to seeding of about 15 per cent. But why the convective clouds in Israel should be more responsive to seeding than rather similar clouds in other parts of the world is far from clear." And why Beijing clouds should resemble Israeli clouds more than those found over Florida is equally obscure.