Saturday, February 20, 2021

The Science of Making Rain

[This is a transcript of the video embedded below]

Wouldn’t it be great if we could control the weather? I am sure people have thought about this ever since there’ve been people having thoughts. But what are scientists thinking about this today? In this video we’ll look at the best understood case of weather control, that’s making rain by seeding clouds. How is cloud seeding supposed to work? Does it work? 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 for increasing precipitation, which is a fancy word for water that falls off the sky in any form: rain, snow, hail and so on. One seeds a cloud by spraying small particles into it, which encourages the cloud to shed precipitation. At least that’s the idea. Cloud seeding does not actually create new clouds. It’s just a method to get water out of already existing clouds. So you can’t use it to turn a desert into a forest – the water needs to be in the air already.

Cloud seeding was discovered, as so many things, accidentally. In nineteen-fourty-six a man named Vincent Schaefer was studying clouds in a box in his laboratory, but it was too warm for his experiment to work. So he put dry ice into his cloud box, that’s carbon dioxide frozen at about minus eighty degrees Celsius. He then observed that small grains of dry ice would rapidly grow to the size of snowflakes.

Schaefer realized this happened because the water in the clouds was supercooled, that means below freezing point, but still liquid. This is an energetically unstable state. If one introduces tiny amounts of crystals into a supercooled cloud, the water droplets will attach to the crystals immediately and freeze, so the crystals grow quickly until they are heavy enough to fall down. Schaefer saw this happening when sprinkles of solid dry ice fell into his box. He had seeded the first cloud. In the following years he’d go on to test various methods of cloud seeding.

Today scientists distinguish two different ways of seeding clouds, either by growing ice crystals, as Schaefer did, that’s called Glaciogenic seeding. Or by growing water droplets, which is called hygroscopic seeding.

How does it work?

The method that Schaefer used is today more specifically called the “Glaciogenic static mode”, static because it doesn’t rely on circulation within the cloud. There’s also a Glaciogenic dynamic mode which works somewhat differently.

In the dynamic mode, one exploits that the conversion of the supercoooled water into ice releases heat, and that heat creates an updraft. This allows the seeds to reach more water droplets, so the cloud grows, and eventually more snow falls. One of the substances commonly used for this is silver iodide, though there are a number of different organic and inorganic substances that have proved to work.

For hygroscopic seeding one uses particles that can absorb water that serve as condensation seeds to turn water vapor into large drops that become rain. The substances used for this are typically some type of salt.

How do you do it?

Seeding clouds in a box in the laboratory is one thing, seeding a real cloud another thing entirely. To seed a real cloud, one either uses airplanes that spray the seeding particles directly into the cloud, or targets the cloud with a rocket which gives off the particles, or one uses a ground-based generator that releases the particles slowly mixed with hot air, that rises up into the atmosphere. They do this for example in Colorado, and other winter tourism areas, and claim that it can lead to several inches more snow.

But does it work?

It’s difficult to test if cloud seeding actually works. The issue is, as I said, seeding doesn’t actually create clouds, it just encourages clouds to release snow or rain at a particular time and place. But how do you know if it wouldn’t have rained anyway?

After Schaefer’s original work in the nineteen-fifties, the United States launched a research program on cloud seeding, and so did several other countries including the UK, Canada, India, and Australia. But evidence that cloud seeding works didn’t come by for a long time, and so, in the late nineteen-eighties, funding into this research area drastically declined. That didn’t deter people from trying to seed clouds though. Despite the absence of evidence quite a few winter sport areas used cloud seeding in an attempt to increase snow fall.

But beginning around the turn of the millennium, interest in cloud seeding was revived by several well-funded studies in the United States, Australia, Japan, and China, for just to name a few. Quite possibly this interest was driven by the increasing risk of drought due to climate change. And today, scientists have much better technology to figure out whether cloud seeding works, and so, the new studies could finally deliver evidence that it does work.

Some of the most convincing studies used radar measurements to detect ice crystals in clouds after a plane went through and distributed the seeds. This was done for example in a 2011 study in Australia and also in a 2018 study in the northern part of the United States.

These radar measurements are a direct signature of seeding, glaciogenic seeding in this case. The researchers can tell that the ice crystals are caused by the seeding because the crystals that appear in the radar signal replicate the trajectory of the seeding plane, downwind.

From the radar measurements they can also tell that the concentration of ice crystals is two to three orders of magnitude larger than those in neighboring, not-seeded areas. And, they know that the newly formed ice-crystals grow, because the amount of radar signal that’s reflected depends on the size of the particle.

This and similar studies also contained several cross checks. For example, they seeded some areas of the clouds with particles that are known to grow ice crystals and others with particles that aren’t expected to do that. And they detected ice formation only for the particles that act as seeds. They also checked that the resulting snowfall is really the one that came from the seeding. One can do this by analyzing the snow for traces of the substance used for seeding.

Besides this, there are also about a dozen studies that evaluated statistically if there changes in precipitation from the glaciogenic static seeding. These come from research programs in the United States, Australia, and Japan. To get statistics, they monitor the unseeded areas surrounding the seeded region as an estimation of the natural precipitation. It’s not a perfect method of course, but done often enough and for long enough periods, it gives a reasonable assessment for the increase of precipitation due to seeding.

These studies typically found an increase in precipitation around 15% and estimated the probability that this increase happened just coincidentally with 5%.

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

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

The world’s biggest weather modification program is China’s. The Chinese government employs an estimated 35,000 people to this end already, and in December 2020 they announced they’ll increase investments into their weather modification program five-fold.

Now, as we have seen, cloud seeding isn’t terribly efficient and for it to work, the clouds have to be already there in the first place. Nevertheless, there’s an obvious worry here. If some countries can go and make clouds rain off over their territory, that might leave less water for neighboring countries.

And the bad news is, there aren’t currently any international laws regulating this. Most countries have regulations for what you are allowed to spray into the air or how much, but cloud seeding is mostly legal. There is an international convention, the Environmental Modification Convention, that seventy-eight states have signed, which prohibits “the military and hostile use of environmental modification techniques.” But this can’t in any clear way be applied to cloud seeding.

I think that now that we know cloud seeding does work, we should think about how to regulate it, before someone actually gets good at it. Controlling the weather is an ancient dream, but, thanks to Vincent Schaefer, maybe it won’t remain 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 and not just talk about it”.


  1. I am not sure how practical this is, or if practical how desirable it is. I was unaware there was a growing interest in this. As you say you need the moisture in the air to start. Clouds have water droplets or ice crystals and these methods involve the introduction of a phase transition inducing particle sites into the cloud to enhance droplet formation. I do not know if this has been done with just plain humidity.

    If you want to really control weather I think one needs to have large spacebased mirrors or Fresnel lenses in space that focus energy to vaporize water that are directed then by wind currents to where you want that. Potentially blocking or light deflecting systems could be used to cool regions. Then from there maybe you seed clouds. That will not help major desert regions, for they are at the boundary of Hadley cells that tend to repel such currents. I wrote a white paper quite a number of years ago about using large numbers of Mie scattering particles or fibers at the L1 point to deflect or scatter a significant enough radiation to lower solar irradiance of the Earth.

    In watching Sci-Fy videos some time back there was a CGI video about these people seeding clouds with airplanes, but they were also fighting air battles with some enemy trying to oppose them. Modifying weather brings about big issues politically. China is a bit notorious on this. With the Himalayan water sheds into the Indus, Brahmaputra, Ganges, Irrawaddy, Mekong and other rivers they have been building dams and reducing water flow. This has a lot of Southern Asia worried. This is in addition to glacier retreats from climate change. If some nation is able to monopolize the control of weather that would pose lots of issues for much of the world.

    1. Monopolize is too strong a word I think, but I agree that there is trouble at the front door. It's clearly doable in principle, we have know that for decades. The question is only how long it will take for a nation to get so good at it that other nations will suffer.

    2. A Pink Floyd put it, "With, without, after all it's what the fighting's all about." There will doubtless be in a future world of climate and weather modification those regions and people who have the assets and those who do not. History shows there is little occasion where that has not been the case.

      The future may see water wars that are far more serious than oil wars.

    3. Water civil wars more likely. Can you honestly believe a non-incorporated group of people can settle on whether there should be raining or not tomorrow?

      Now, for a incorporated one ... suddenly Facebook looks like innocent joke.

  2. Back when one of my brothers owned 320 acres of land near the hamlet of Donald, Ontario, we would spend holiday weekends at the cabin we built alongside a river. On one of these wintertime visits the temperature dropped to -20 Fahrenheit (-29 Celsius). Exhaling warm, moist breath into such extremely cold air would create a mini snowstorm. It was kind of neat to watch the tiny, crystallized snowflakes gently fall to the ground.

  3. It's good to see that the atmosphere is being investigated and tested. Richard Feynman put forth a challenge to physicists in the'50's to unlock the mystery of the enormous energies found in the atmosphere but that has not been a success as far as I know.
    The atmosphere remains a deep mystery with all sorts of strange things besides energy flows. Why does water vapour form distinct clouds and not a diffuse vapour state? What keeps the atmosphere pinned to earth's surface as it rotates at phenomenal speeds? Why is the ration of N2 to O2 so constant?
    Maybe easier to hypothesize about multiverses than about something that can be

  4. Quanta Magazine: Cloud-Making Aerosol Could Devastate Polar Sea Ice.

  5. A very interesting topic! Following in the footsteps of Lawrence about aerosols and other pollutants. I've noticed that when passenger planes fly over clouds at low altitudes (mostly stratus type comes to mind) they seem to disappear. And that happens even when the plane is high enough above them.

    I was intrigued by that phenomenon but could not find what happens. As most information in search was about some other conspiracy theory (chemtrails, or something like that). And it seemed that some process of condensation is at work (though it was not obvious for big difference in altitudes, and if memory serves me correctly, some clouds seemed to be dissipated just before the plane flew over them).

    And recently I've discovered an article in ScienceMag around a similar topic (on higher altitudes) about contrails, aircraft routing and its surprising connection to the climate change. It claims that soot makes water vapor to condense and form cirrus clouds behind at high altitudes, thus forming the contrail. Perhaps, it has the opposite effect for clouds on lower altitudes that lie beneath the plane, so that vapor condenses and clouds disappear (just a guess, or might it be something connected with sound waves propagation). Anyway, here is the article:

    There is another correlation between the global warming and cumulus clouds thinning as it's more difficult for vapor to stay supercooled:

    I hope this topic will find further interest with viewers/readers as it's mind-boggling to think how it all works (rain, precipitation, cloud formation, wind currents, etc.).

    PS It seems simulation/free-will/flat-earth/consciousness category is missing another heavy hitter - UFOs!

    PPS Also, how does YT algorithm work (if someone knows)? Is it satisfied with "Like" or it needs some bells, uninterrupted views & comments (engagement)?

    1. Hi Vadim,

      Thanks for those references. Interesting topic indeed, I might look into this.

      I suspect no one besides YT knows how their algorithm works, but I know as much as that it doesn't just count views, it keeps track of how much people watch of a video. I don't know how much comments matter.

    2. Ha-ha, thanks! That was an interesting concurrence with Ozone video.

      PS Concerning the algorithm, I've seen many opinions (yeah, not the best source of information) that it takes comments (and even comments likes) as input (in many hot topics it was suggested to hit 5-7 likes to comments to raise videos). FWIW.

    3. Sabine, another topic came to mind, which is at the junction of popular culture and science - everything is connected. Suzanne Simard studied (paper & all) how trees in forest are interconnected by mycorrhizal networks. So that forest literally operates its own Internet on different levels (hands down, the most beautiful thing I've re-discovered last year).

      She seems to be underrated (notwithstanding some attention). Perhaps, Suzanne might be a good candidate for an interview.
      Seminal paper:
      Article at NYTimes:

    4. Fungi are obligate parasites and infect other life forms to survive. Crops e.g. corn are blighted by fungi. Of course the 'truth' is what we would like it to be as long as we area able to claim consensus (enough support)

    5. Not sure what you want to express. Calling something parasite doesn't explain anything as there are obviously different fungi and they perform different functions in different environments. In the same vein, we may call all bacteria 'evil', 'good' or with any other empty word. It's all about the structure of relations and function each organism performs. Then, how it's called is of secondary nature.

      What I enjoy in Suzanne's work the most - it shows how interconnected all processes really are. Their relations, and how complex the functions of everything what we call alive are. How mycorrhizal networks and mycelium operate and trees communicate in its truest sense. Making forest truly a living-breathing organism.

      "the 'truth' is what we would like it to be as long as we area able to claim consensus"
      That's a common misconception. Truth does not care what I or you, or anyone else, thinks. It's we who are better to start caring.

      Earth revolving around Sun does not care if there ever would be some Aristarchus (300 bc!) or Copernicus. Yet, when Copernicus tells us (however big and 'knowledgeable' this 'us' is) about it, he has something beyond his belt that doesn't care one bit about me, you, the multitude and the consensus. That's what we call fact. It is so.

      Our previous descriptions and interpretations of this fact (for Ptolemy and scientists, for laymen it's just an another belief) led us into epicycles and other difficulties. Another scientist later gave it better description. Yet, the math of epicycles is still working to compute orbits from Earth exactly because it was *not a belief* on part of Ptolemy. It's just he used the best knowledge he had at the moment to build the structure, the model.
      Meeting Copernicus in 15th century he would instantly understand data and change his model in a day or so. While to a layman it *still* does not matter whether Sun is a disc over head, some funky god, or whether it revolves around Earth or not. Meeting Copernicus or anyone else, for that matter, would not change anything. That's a belief.

      Doxastic and scientific knowledge (opinions/beliefs and facts) are not on the same footing. Truth is not what consensus wants it to be. To clarify this moment once and for all one can *even* read "Gorgias" by Plato! Read Feynman's "What is science?" and "Cargo Cult Science" Or... Sabine's very own fantastic video explaining the difference:

      PS Gosh, it was a strange week! As this exact moment keeps popping up everywhere in communication with people around (what's more surprising some even went through some STEM training - programmers, math folks, people with some 'physics' (!) training, etc. - and only now suddenly started to speak up what they think... and it's this).
      Seriously, I myself make similar and noob mistakes all the time. And, trust me, many people on this forum are *very* patient and kind. But this specific one - difference between belief and scientific knowledge - it's time for it to go to as one of the biggest humiliations of mankind that has ever existed (probably, second only to 'killing each other') into its "shameful history".

    6. Parasite is not just a word. It is a process in which a given life form extracts energy from another life form (host) to a degree that is greater than the amount that it contributes, resulting in negative consequences for the host. Fungi are obligate parasite by definition.
      Truth is a slippery term. For any theory at all, proof i.e. truth is the ability to create or fundamentally change the subject of the theory. Such proofs do not exist. It is not possible create the hypothesized situation. Truth is declared when an effective consensus is reached. Provide a counter example if you can.
      Of course theories can be important in the sense that “ all theories are wrong but some are useful"

    7. "It is a process in which a given life form extracts energy from another life form (host) to a degree that is greater than the amount that it contributes"
      By that definition, Homo is one. (btw, fungi are heterotrophs, that was not the point).

      "Truth is a slippery term."
      It was not used as a term. We use language to describe relations, assuming the structure of phenomena is somewhat understood (model). Also, theory is not what you think it is. The rest is already described.

  6. Yet another urgent and interesting problem few people have heard or are interested in (certainly not the hype type) - water management.

    I've been wandering through Tamil Nadu & Kerala (in 2016), people were talking about a severe draught that year (if not mistaken, it was even worse the next year). They had a severe argument about water supplies, as Kerala has dam on a river they share, and can regulate the level of water. Which, as I understood, turns ugly in years with low levels of precipitation during the rainy season. Seeing riverbeds dry all along the land was scary.

    I talked with some people who tried to revive the earth in central parts. It was long and arduous process, which included growing resistant to aridity plants. They had some success and began to grow some agricultural plants and vegetables.

    As I understand it's one of the major parts of equation - that the earth must have the capacity to hold water (e.g. there are some mistakes with irrigation of swamps that Soviets did and it led to desertification of good land, as swamps played the function of water capacitor of sorts).

    So I'm not sure the solution you've mentioned might work as the problem (which is enormously complex in itself, so it's clear that all this is just some random ramblings) contains at least three components - groundwater (in India, it's bad in the SE, and better in SW), soil capacity to hold water (plants, etc.), and clouds. It's only clear that it has to be approached in some composite way (not by drainage of Ganga and similar means) and the soil has to be taken care of beforehand (that's the serious contradiction).

    I've since seen an interesting article on the topic approaching the problem with water supplies differently, so it may be of interest:


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