What is cloud seeding and could it end the drought in Iran?

Iran is experiencing a drought so intense that the country’s president has said the capital, Tehran, may need to be evacuated. In an attempt to bring rain, planes launched a cloud-seeding operation on November 15 that is expected to last through the traditional rainy season until May. But experts warn that the technique is difficult and unlikely to contribute to a major breakthrough in the water crisis on its own.

How bad is the drought in Iran?

Rainfall in Iran is 85% below average and Tehran has received only a millimeter of rain this year. Reservoirs in the capital and neighboring regions are in a “worrying condition,” officials said, with water capacity below 5 percent at 32 dams. Satellite images reveal that some are completely dried up.

Tehran residents have reportedly reduced their water consumption by around 10 percent, but this is not enough, according to authorities. Water pressure has been reduced at night and authorities plan to impose fines on households and businesses that use too much water. If rainfall does not resume by December, Tehran’s 14 million residents may have to start evacuating, the president said.

What is the cause of drought?

Experts say the climate crisis has contributed to the drought, which has already lasted five years. Iran is experiencing its driest autumn in 50 years, and Tehran, which often receives snow in November, is experiencing temperatures of 15°C (59°F) or higher.

But poor management is the main cause of what Kaveh Madani, a former environment ministry official, calls Iran’s “water bankruptcy.” The government has massively expanded agriculture in arid areas, overtaxing water resources. Half a million illegal wells, many dug by desperate farmers, have depleted groundwater.

What is cloud seeding?

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

However, it is often difficult to know how much rain or snow there would have been without cloud seeding.

“The effects are very difficult to demonstrate due to the high natural variability of clouds,” says Andrea Flossmann of Clermont Auvergne University in France. “You look out, you have a field of clouds, and there are some clouds that rain, and some that don’t rain.”

A 2014 experiment comparing two Wyoming mountain ranges found that cloud seeding could enhance precipitation by 5 to 15 percent.

Can it solve drought?

Iran, which previously accused Israel and the United Arab Emirates of stealing its rain by seeding clouds, now has its own program that involves spreading seeding agents from cargo planes, drones and “ground generators,” a term that generally refers to smoke furnaces located high in the mountains.

He said he sowed clouds on Nov. 15 in the basin around Lake Urmia, which over two decades had dried up and become a salt plain littered with rusting boats. Areas west of the lake received up to 1 inch (2.7 centimeters) of rain early the next morning, according to a precipitation map compiled by the University of California, Irvine.

However, for a cloud seeding campaign to aim to replenish reservoirs, clouds must contain a lot of water. This type of cloud can be difficult to find in arid Iran, where there aren’t many large bodies of water to evaporate moisture into the air.

“Cloud seeding is often much more difficult during a drought because the atmosphere is very dry and the clouds present may not have enough moisture,” says Karen Howard, a scientist at the U.S. Government Accountability Office.

But masses of rain clouds fell on Iran from the Black Sea over the past three days, even causing flooding in western provinces including Ilam and Kurdistan on November 16.

Cloud seeding will at least “squeeze a few extra drops” out of weather systems like this, says Armin Sorooshian of the University of Arizona. “This will not lead to extreme situations like flooding or resolve widespread drought,” he says. “But it might help a little.”