Fact file: Cloud seeding

WATCH: How scientists are manipulating the weather through cloud seeding

TORONTO – ‘Cloud seeding’ is being credited with saving millions of dollars in hail damage in Calgary on Sunday night. But what is this technique, who uses it, and how do we know if it works? explains the technology of cloud seeding.

What is it? Cloud seeding is a type of weather modification that can enhance a cloud’s ability to produce precipitation, according to American-based company Weather Modification Inc. Cloud seeding is used in three areas: trying to increase precipitation, disperse fog, and-most recently in the case of Calgary’s storm Sunday evening-attempt to mitigate hail damage using an aircraft.

How does it work? In aerial cloud seeding to mitigate hail, planes fly into the cloud area and shoot silver iodide at the top and base of the clouds. Silver iodide is a “seeding agent” which has a chemical structure similar to that of ice and can help shrink the ice stones or induce the formation of ice.

Aerial cloud seeding is typically the most effective way to target a cloud since the aircraft gets as close as possible. Equipment includes burn-in-place flare racks or ejectable flare racks attached to the aircrafts. The operation can last for many hours-12 in the case of Calgary’s hailstorm.

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Environment Canada senior climatologist Dave Phillips says the Calgary operation was carried out to produce hail closer to the size of peas rather than baseballs.

“Pea-sized hail does very little damage, whereas golf ball or baseball-sized hail can be lots of damage,” says Phillips. “They’re trying to take the amount of water vapour in the air and spread it around to more nuclei.”

Another cloud seeding application is to cause rain, with the aim of encouraging precipitation in one location and avoiding another. In this situation, the basic principle is that the tiny water drops that form a cloud are too small to fall down to the ground on their own because of the rising air current. The cloud seeding technology can shoot crystals into the cloud (from the ground as well as from the air) so that the water drops coagulate and become bigger drops. Bigger drops are heavier and the rising air current won’t be able to support their weight; therefore they fall as rain.

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How much does it cost? Weather Modification Inc., the company responsible for cloud seeding during Calgary’s recent storm, says the project cost depends on factors such as target area size, season, needed equipment and length of desired project period. Dr. Terry Krauss, project director of the Alberta Severe Weather Management Society, says their budget was $2.5 million for this year. These funds come from the private insurance companies of Alberta through a program designed to reduce property damage. Krauss adds the previous record hail storm in Calgary was July 12, 2010 when damages exceeded $400 million.

“The economics are what is driving the program and if we happen to reduce the hail on Sunday’s storm by even 10 per cent, that could have saved $50 million and paid for the program 20 times over,” said Krauss in a phone call to

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Who does it? Fifteen-year-old American-based company Weather Modification Inc. was hired by the Alberta Severe Weather Management Society on Sunday (a society funded by a group of insurance companies with a goal of reducing hail damage claims).

Cloud seeding companies exist in the U.S., Europe, Australia, Africa and Asia. Asia is home to the largest cloud seeding systems in the world in the People’s Republic of China.

Where has it been used before? A notable previous use was at the 2008 Summer Olympics in Beijing to ensure optimal weather during Opening and Closing Ceremonies. According to Chinese state media, cloud seeding kept rain from hitting the stadium by causing the rain to fall elsewhere.

How effective is it? Weather Modification Inc. says success can be measured using precipitation data comparing rainfall or snowfall during comparable seeded and non-seeded periods of time or locations.

However, senior member of the Chinese meteorological Association Johnny Chan told BBC Radio following the Beijing Games that despite being tested since the 1950’s in the U.S. and Russia, “sometimes it works, sometimes it doesn’t.”

In order to test whether or not the operation really makes it rain, you’d have to have two identical clouds – one that you would cloud seed, and one you would leave alone as a control. Then if the seeded cloud rained but the other didn’t, you’d know that the technology worked. Since that’s not possible, there’s no way to scientifically prove its effectiveness.

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In the case of the Calgary hail storm-where cloud seeding was credited for sparing the city from even worse hail damage-there’s no way to tell how much milder or more severe the hail would have been without the operation. However, Krauss says radar data indicated the storm was more severe before it entered the city limits prior to the start of seeding, and the resulting regions of damage were more sporadic than if there had been no cloud seeding.

Could cloud seeding be used as a weapon? Chan noted that it’s possible that you could rob locations of water, causing a drought by making rain fall outside a country’s borders instead of inside. This could potentially start a war, but in 1977 an international treaty banning the use of weather modification for hostile purposes was signed by Canada, the U.S. and 31 other countries.

What is the environmental impact? Weather Modification Inc. says numerous studies have shown the safety of silver iodide, a compound they consider “stable, essentially inert.” Their website adds that the effects of cloud seeding on precipitation are around 10 per cent, a number they call “negligible.”

Phillips adds that such miniscule amounts of silver iodide are used that they have no effect on human life, drinking water or the environment at all, likening the operation to “the proverbial drop of dye in a lake.”


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