[This is a transcript of the video embedded below]

Wouldn’t it be great if we could control the weather? I’m sure people have thought about it for as long as there have been people who have thoughts. But what do scientists think about it today? In this video we look at the best understood case of weather control, where rain is created by sowing clouds. How should cloud seeding work? It works? And if it works, is it a good idea? That’s what we’ll talk about today.

First things first: what is cloud seeding? Cloud seeding is a method of increasing rainfall. This is a fancy word for water falling from the sky in some form: rain, snow, hail and so on. You sow a cloud by spraying small particles into it, which stimulates the cloud to throw off precipitation. At least that’s the idea. Cloud seeding does not create new clouds. It is just one way of getting water out of clouds that are already there. So you can’t use it to turn a desert into a forest – the water must already be in the air.

Like so many things, cloud seeding was accidentally discovered. In 1964 a man named Vincent Schaefer was examining clouds in a box in his laboratory, but it was too warm for his experiment to work. So he put dry ice in his cloud box, which is carbon dioxide that is frozen at around minus eighty degrees Celsius. Then he observed that small grains of dry ice would quickly grow to the size of snowflakes.

Schäfer realized that this was happening because the water in the clouds was supercooled, that is, below freezing point, but was still liquid. This is an energetically unstable state. When tiny amounts of crystals are placed in a supercooled cloud, the water droplets will immediately adhere to the crystals and freeze, causing the crystals to grow quickly until they are heavy enough to fall off. Schäfer saw this when a splash of solid dry ice fell into his box. He had sown the first cloud. In the following years he tested various cloud seeding methods.

Today scientists distinguish two different ways of sowing clouds, either by growing ice crystals, as Schäfer did, which is known as glaciogenic sowing. Or through the growing of water droplets, which is known as hygroscopic sowing.

How does it work?

The method used by Schaefer is now more precisely referred to as “glaciogenic static mode”, static, as it is not based on the circulation within the cloud. There is also a glaciogenic dynamic mode that works a little differently.

In dynamic mode, one takes advantage of the fact that the conversion of the supercooled water into ice releases heat and that heat creates an updraft. This allows the seeds to reach more water droplets, causing the cloud to grow and eventually more snow to fall. One of the commonly used substances for this is silver iodide, although a number of different organic and inorganic substances have been found to be effective.

For hygroscopic sowing, particles are used that can absorb water, which acts as condensation seeds to convert water vapor into large droplets that turn into rain. The substances used for this are typically some type of salt.

How you do that?

Planting clouds in a box in the lab is one thing, planting a real cloud is another thing. To sow a real cloud, one either uses airplanes that spray the seed particles directly into the cloud, or a missile aimed at the cloud that emits the particles, or one uses a ground-based generator that slowly mixes the particles with hot air releases, that rises into the atmosphere. They do this, for example, in Colorado and other winter tourist areas and claim that it can lead to several inches of more snow.

But does it work?

It is difficult to test that cloud seeding actually works. The problem, as I said, is that sowing does not produce clouds, only clouds are stimulated to release snow or rain at a certain point in time and in a certain place. But how do you know if it wouldn’t have rained anyway?

Following Schaefer’s original work in the 1950s, the United States started a research program on cloud seeding, as did several other countries including the UK, Canada, India and Australia. However, there has been no evidence that cloud seeding works for a long time, and funding for this area of ​​research dropped dramatically in the late 1980s. However, that didn’t stop people from sowing clouds. Despite a lack of evidence, some winter sports areas used cloud seeds to increase snowfall.

However, from the turn of the millennium, interest in cloud seeding was revived through several well-funded studies in the United States, Australia, Japan, and China, to name a few. Possibly this interest was due to the increasing risk of drought due to climate change. And today, scientists have a much better technology for finding out if cloud seeding works, and so the new studies could finally provide evidence that it works.

Some of the most compelling studies used radar readings to detect ice crystals in clouds after an airplane passed through and dispersed the seeds. For example, this was done in a 2011 study in Australia and also in a 2018 study in the northern United States.

These radar measurements are a direct signature of the sowing, in this case the glaciogenic sowing. The researchers can determine that the ice crystals are caused by the sowing, since the crystals appearing in the radar signal replicate the trajectory of the sowing surface in front of the wind.

Using the radar measurements, they can also see that the concentration of ice crystals is two to three orders of magnitude higher than that in neighboring, unsown areas. And they know that the newly formed ice crystals grow because the amount of reflected radar signal depends on the size of the particle.

These and similar studies also contained several cross-checks. For example, they have seeded some areas of the clouds with particles that are known to form ice crystals and others with particles that are not expected to. And they discovered ice formation only for the particles that serve as seeds. They also checked that the resulting snowfall is really the one that came from the sowing. This can be done by analyzing the snow for traces of the substance used for sowing.

There are also about a dozen studies that have statistically evaluated whether the precipitation changes due to the glaciogenic static sowing. These come from research programs in the USA, Australia and Japan. To obtain statistics, they monitor the unseeded areas surrounding the sown region as an estimate of natural precipitation. It is not a perfect method, of course, but often enough and for sufficiently long periods of time there is a reasonable estimate of the increase in rainfall due to sowing.

These studies typically found a 15% increase in precipitation and estimated the probability that this increase was accidental at 5%.

For at least the seeding of ice crystals, there is now pretty solid evidence that it works better than a rain dance. It is still unclear whether the other types of sowing are efficient.

Please check the information below the video for references to the papers.

The world’s largest program for changing the weather is that of China. The Chinese government already employs an estimated 35,000 people for this purpose and announced in December 2020 that it would increase investments in its weather change program by five times.

As we’ve seen, cloud seeding isn’t particularly efficient, and for it to work the clouds must already be in place. Even so, there is an obvious concern here. If some countries can rain clouds over their territory, there may be less water left for neighboring countries.

And the bad news is that there are currently no international laws that regulate this. Most countries have regulations about what or how much you can spray into the air, but sowing clouds is mostly legal. There is an international convention, the Environmental Modification Convention, signed by seventy-eight states, which prohibits “the military and hostile use of environmental modification techniques”. However, this cannot be clearly applied to cloud seeding.

I think now that we know cloud seeding works, we should think about how we can regulate it before someone gets really good at it. Controlling the weather is an old dream, but thanks to Vincent Schaefer it may not stay a dream forever. When he died in 1993, his obituary in the New York Times said, “He was hailed as the first person to actually do something about the weather, not just talk about it.”



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