New Scientist:

By Alec Luhn

Cloud seeding was developed in the 1940s by scientists including Bernard Vonnegut, brother of the novelist Kurt Vonnegut. It involves dispersing particles that encourage the suspended water droplets in clouds to fall as rain. Although some projects have sprayed salt into low-lying clouds, many have focused on spreading chemicals, most commonly silver iodide, into higher, mixed-phase clouds. The supercooled liquid water droplets freeze on contact with this crystalline compound, forming ice crystals that grow heavy and fall as snow or rain.

It is often hard to know how much rain or snowfall would have occurred without the cloud seeding, though.

“The effects are very difficult to show because of the large natural variability of clouds,” says Andrea Flossmann at University Clermont Auvergne in France. “You look outside, you have a cloud field, and there are clouds that rain, and others don’t rain.”

An experiment in 2014 comparing two mountain ranges in Wyoming found that cloud seeding could strengthen precipitation by 5 to 15 per cent.

Can it solve the drought?

Iran, which previously accused Israel and the United Arab Emirates of stealing its rain through cloud seeding, now has its own programme that involves spreading seeding agents from cargo planes, drones and “ground generators”, a term that typically refers to smoke furnaces on high mountains.

It said it seeded clouds on 15 November in the basin around Lake Urmia, which, over two decades, has dried up into a salt plain littered with rusty boats. Areas west of the lake received up to 2.7 centimetres of rain early the next morning, according to a precipitation map run by the University of California, Irvine.

For a cloud-seeding campaign to replenish reservoirs, however, the clouds must contain a lot of water. That kind of cloud may be hard to find in arid Iran, where there aren’t many large water bodies to evaporate moisture into the air.

Go to link