Can we stop the rain?

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0:59That's where your story starts.

1:12I'm wandering around Mumbai in a neighborhood called Wadala, where two things are happening. First, there's a religious festival in full swing. And second, it's raining. Really raining. The monsoon has arrived.

1:30This celebration is called Devshayani Ekardashi and it marks the day that Lord Vishnu goes into a four-month slumber and the changing of the seasons and the start of the monsoon. It comes once a year, so it comes during monsoons and there is a folklore which says that there was no rains and they prayed for it.

1:52The monsoon typically begins in southwest India around late May or early June and moves north. By mid-July, the rains are across the whole country.

2:06A little to the west, on Dadar Beach, people don't seem to mind the rain. There are people playing soccer and beach volleyball, all drenched to the skin, happily playing on regardless. The rain is soft and warm and everything is sparkling and green. Oh, and it's raining really hard. I should get back under some shelter. This is Crowd Science from the BBC World Service, the show where we take your questions about life, the universe and everything and get you answers from experts. My name is Shabhi Sajdeev and I have to admit it, I love the rain. And I'm not the only one. Around 150 kilometers from me, in the city of Pune, there's a crowd science listener who also loves the rain.

2:53Well, sometimes, anyway. It has a mind of its own, I think, the rain. And it can come anytime, it can go anytime. This is Rit. We met on a balcony overlooking a verdant garden in Pune's Koregao Park. In the morning it was raining and there was a cloud burst here. Can you hear that? We can hear peacocks where we're sitting. Yes. Peacocks are supposed to be like the harbingers of the monsoon. Yes, yes. We're sitting in the lap of nature, I think. So what was the question you wrote into CrowdScience? Could you read it out, please? Dear CrowdScience, the monsoon rains have come to us here in Pune, India, and we haven't seen the sun for almost four days now.

3:38It's not fun anymore, not just because all my shoes are wet, but also because it's hard on the animals and it's destroying the crops also. My question is, can we artificially stop rain pour in the case of emergency? So tell me a little bit about your experience of the monsoon. Do you look forward to it or dread it? I actually like rain. The smell of the mud comes and it's a lovely smell, I will say. As a child, I would look forward to the monsoon because it would mean my schools are off because of rainy days. Also at times we would go out and play football in the rain. That's a fun thing. That's a fun thing.

4:21But I have a lot of cats and animals and I think that cats and animals do find it very hard when there is a heavy downpour. These are outdoor cats? Outdoor cats but we feed them every day. During rain though they don't come and during the storms and they definitely fight for life. Also these crops and agriculture, they rely on rain. So I think that rain is actually beneficial, it's not bad. Rain is absolutely necessary, but there is a threshold that if it crosses it then it can become destroying, it can become bad. That's why I asked the question that in case of emergency we can somehow weather engineer it to put rain in that threshold limit and not make it exceed.

5:07maybe even control where it goes to. Actually, yes, if cloud seeding can bring rain, I definitely think that there is some process that can stop rain also in case of emergency. Can we stop the rain? Good question, Reet. And it's an important one too, not just for the sake of Reet's poor hungry cats, but also for national agriculture. Excess rainfall during the monsoon is reportedly responsible for 5.7 billion US dollars worth of damage in India every year. Wheat, rapeseed, cotton, bananas, a lot of crops suffer. I did a story for our sister program, People Fixing the World, a few years ago, where I met tomato farmers whose entire crop was worthless because rain had damaged everything.

5:57And around the world, we're seeing more cloud bursts, flash floods and storms as our climate warms up. I've come just a little way west of the city to the Indian Institute of Tropical Meteorology to meet someone who can help me find out if that's possible. It's not raining when I arrive, but you can tell it has been. The grounds of the institute are like a tropical paradise with banyan trees, bamboo groves and masanda and franjipani just bursting with blooms. clouds of dragonflies are flitting across the lawns. I'm here to meet scientist Dr. Tara Prabhakaran, who studies cloud physics and weather engineering.

6:42Tara walks me through the facility to her office. On the way, we pass a model of the campus, complete with a tiny model airplane, radar station on the roof, and fluffy little grey clouds bursting with rain. You can see the aircraft and the cloud and the rain. No, no, it is not to scale. It is a small compared to that. I am Tara Prabhakaran. I am working as a scientist in Indian Institute of Tropical Materiology, Pune. I look at clouds, basically. That is my real, very dear topic. Although we think that the rain forming is a very straightforward mechanism, it is not so. So it's a very complex mechanism.

7:28We have to understand how the clouds are formed, how do they grow, and how they make rain. So is it fair to say you like the rain? Yeah, yeah, I like the rain. Even watching rain, these drops falling, is a very fantastic thing. Tara is the director of Kypex, or to give it its full name, Cloud Aerosol Interaction and Precipitation Enhancement Experiment. It's a very long name, but there are two different aspects. to it. Kypex has been doing groundbreaking, or perhaps I should say cloud bursting research, since 2009. And their goal is not just to understand how clouds work, but also how we can use technology to change what they do, to make it rain.

8:14You've probably heard of the technology, it's been around a while. It's called cloud seeding. Now, cloud seeding is a technology that's all about starting the rain, which is the opposite of what Reet was hoping to do. But if anyone knows how to make a cloud behave the way we want it to, it's Tara. Perhaps she knows how to stop the rain too. We are trying to understand how the cloud seeding, how it works, what is the mechanism which makes it rain, whether it works or not. The science of clouds is, as Tara says, pretty complex. But let me give you a basic rundown. The air contains moisture. For that moisture to form into a cloud, it needs to condense, either into tiny water droplets if it's warm enough, or tiny ice crystals if it's cold.

9:03Now, these droplets don't form all by themselves. They need a surface to form upon. Luckily, there are tiny particles called aerosols floating in the air. Things like dust, sea salt, ash, or even pollen. Micrometer-sized particles. We are talking about our hair thickness, right? Even human-made pollutants like soot or agricultural chemicals, they're all up there in the air. So these particles are important in the formation of cloud, whether they make it rain or not. Those tiny particles are like the grit in the oyster, which forms the pearl. The water in the air condenses around these particles, and then those tiny droplets grow and grow and grow.

9:47When the small drops come together, they collide and they make bigger drops when they fall. That indeed initiates the rain. Or snow, or hail, or whatever other kind of weather you might be having. These aerosol particles are really important to cloud physics. They are referred to as cloud condensation nuclei. We know that, OK, if we put these particles in the atmosphere, they will definitely form droplets. By seeding, we are introducing additional particles, additional seeds for the cloud. Cloud seeding works by distributing new particles into existing clouds to try to encourage this process. At Kypex, they use salts, calcium chloride or sodium chloride, better known as regular old table salt.

10:36In colder climates, they use silver iodide, which is better at forming ice. All these particles are extremely hygroscopic, which means they're great at attracting water out of the air. Tara, though, is not just an expert in the theory. She's been right into the clouds to watch it happen. It was a very laborious exercise. Once we know that there is likelihood of clouds, then we will wait for the radar observations to come. And then, based on that, we will plan our experiment. I am flying scientists who will decide what to do. In an aircraft, we will go to the base of the cloud. We have two people, scientists and engineers, and two pilots.

11:21We will inspect the cloud with our instruments. Once all the measurements have been taken and everyone is happy, they can start the actual seeding process. So these seed particles are in a compressed tube on the wings of the aircraft, several of those flares and it will burn and it will make the fumes of aerosol particles. Once we are finished with this exercise of seeding then we will go to some other cloud like that it continues. You know in a day we will be flying four and a half hours. So over two years, we have done 267 times. And I'll tell you this much, no other country has done. So what are the results of all this experimentation?

12:12Does cloud seeding work reliably? And if it can be used to bring the rain, might it be possible to use it to stop rain too? In a 100 by 100 kilometer square area, we can make 18 % enhancement in these samples that we have conducted. That is pretty encouraging actually. So our listener, our listener Reet, his question was whether we can stop the rain once it starts. Is that possible? Stop rain has not been very much in demand. More resources are definitely spent on making rain. Overseeding, actually overseeding means putting more aerosols into the cloud. There could be a possibility. Overseeding is where the scientists make a little miscalculation and put too much of the seeding particles into the clouds.

13:06Then instead of making nice heavy droplets that fall as rain, this makes far more, much lighter droplets. And they never fall at all, or they evaporate into thin air. I can tell you many clouds we seeded also evaporated. So we know that overseeding can indeed dissipate clouds. It could be a possibility, but we have not tried it. Well, it sounds like it could go either way. It could make it rain harder. Yeah, it could make rain harder somewhere else. That is a possibility. So could you potentially use cloud seeding to make it rain somewhere else instead of the place where it's causing problems?

13:45Yeah, there are some of the research projects are doing some efforts. that. But even for that also we should understand the system and we have to get it right. Okay, the location, intensity and the duration of that storm, we have to get it right. So it's complex system. These areas need a lot of further investigation and research on. In a sentence, can we currently make rain stop? I wouldn't say yes, yet. I think it needs further research. If we follow science, maybe something is possible. Maybe some targeted manoeuvring of the clouds, like not here today, clouds go 300 kilometres and then rain.

14:35That again, it is very complex. I don't want to be that much optimistic on this. But seriously, there are places that don't get enough rain, while there are places that get too much rain, it would be so good to balance it out. Yeah, but the nature also is playing a very big role in that, right? Too much of engineering may not be that working solution.

15:01Perhaps Tara is right to sound a note of caution here. Although weather modification has been around for many decades now, it's always been just a little controversial. The general public are sometimes a little suspicious of the idea of messing with the weather, firing chemicals into the air, or deciding who gets rained on and who doesn't. So far in the show, we've heard a lot about how to make it rain, but we're not making much progress in finding out how to stop it. Should we be doing it at all? That's what we'll be finding out next.

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16:36You're listening to CrowdScience from the BBC World Service, the show that takes your questions and carefully plucks the raindrops of good science out of the dark clouds of confusion. I'm Chavi Sajidev, and today I'm on a quest from listener Reet in Pune, India, who wants to know if it's possible to stop the rain. One of the places that's been controlling the clouds to make rain the longest is the United Arab Emirates or the UAE. Here in the UAE, as one of the world's most Arab countries, I can say we are receiving less than 100 mm of rainfall annually. We have to put a very high consideration on any techniques that provides us with fresh water.

17:16And one of these techniques is the rain enhancement. This is Alia Al-Mazouri, the director of the UAE Research Program for Rain Enhancement Science at the National Center of Meteorology in Abu Dhabi. I've been here for the last 22 years, I can say. Wow. The three components of the UAE rain enhancement program I'm in charge of. The UAE has been working on rain enhancement for decades, and today the country is one of the most advanced in cloud seeding tech. We can go back more than four decades. In those early years, the work was largely experimental more, but it was lacking a lot of strong scientific foundation.

17:57This groundwork would eventually become the UAE Research Program for Reenhancement Science. Pushing the scientists, the researchers all over the world to come, please, and give us more innovative ideas. Let's push the boundaries of the science, look at state-of-the-art techniques, new materials, and fill in the gaps in this field. How successful has it been? Like, how much more rainfall are you seeing? Yeah. According to the peer-reviewed studies by the WMO and a recent Nature Review paper also, rain enhancement can increase rainfall by 5 to 25 percent within 15 to 20 minutes of seeding. One of the areas they're exploring is in nanotechnology.

18:37They've taken the sodium chloride, the ordinary table salt that's often used in cloud seeding, and they've coated it in a microscopically thin layer of titanium oxide. This should make it even more hygroscopic or moisture absorbing than salt on its own. This nanomaterial proved through experiments that it's better in terms of the effectiveness than the natural salts by around three times. And of course, these studies showing no significant environmental or health risks at current usage levels. Because using normal salt, the concern is that it's going to fall back into the earth and will be very bad for agriculture and aquatic life or even terrestrial life.

19:18Yeah, this is one of the myths that are really around the cloud seeding operation. Oh, really? Yeah. We collect different samples over different parts of the UAE that experience cloud seeding operations. And the laboratory results did not reveal any trace level of seeding agents in both surface soil or water bodies. The potential environment impacts of cloud seeding materials have been studied since the 1960s, by the way. And in a recent review by the WMO, the World Meteorological Organization, the expert panel did not find any evidence of negative environmental impacts in more than 30 papers, like 31 papers reviewed.

19:58Since we're talking about the impacts and especially environmental impacts, aren't there concerns that if you engineer a cloud to rain, say over you, are you then having a cross boundary effect? Are you denying a neighboring country that rain? Are there legal, international considerations that also come into play? Yeah, this is also one of the criticisms around the rain enhancement. But I can say if we look at it scientifically, this is not possible because when you come to the lifetime of the cloud, it's around 40 minutes, 45 minutes. When you target this cloud in our airspace, for example, it will affect the area that we are targeting, not other areas.

20:45Although the chemicals used in cloud seeding are dispersed in very low concentrations, there has been some concern from the scientific community about the possible accumulation of these substances in the soil and water. More on that later, but I am curious about what the public thinks too. The public, of course, is very important also to be aware of what we are doing. Now, since we started in the 80s and early 90s as well, a lot of wrong information came out. And one of the most important pillars there is to build the awareness of the public and to get them, of course, aware of the positive impact of this technique, to show them how these materials are really environmentally safe, to show them the impact of that in terms of water security of the country.

21:34Right. I'm just thinking back to 2016 when there were floods in Dubai. And I think it was conclusively proved that cloud seeding did not cause the floods. Yes, yeah, yeah, exactly. But there was so much speculation linking extreme weather events with cloud seeding efforts. Can cloud seeding actually contribute to such intense rainfall or flooding at all? Of course not, of course not. And we had also another case than 2016. We had a case in 2024. And this was one of the things that came out, the negative impact of rain enhancement and how cloud seeding affected and causes that, where really if you talked in a scientific language, you can explain this easily.

22:18These are severe conditions. This is a supercells that came out. You cannot really interfere in this condition. With cloud seeding, you cannot, like the maximum thing, as I mentioned, is like a 25 % increase in terms of the amount. So we do have a limit with that. We have a limit with the percentage of the enhancement for the timing doesn't exceed a couple of hours, of course. Not more than that. Yeah. So the UAE rain enhancement program has nanotech, autonomous drones and AI at its fingertips. Surely they've got something up their sleeves that can help with Reid's question. Is there anything that can stop the rain?

22:57Okay, with our current understanding of physics, technological capabilities. There is not any current technology can stop rain once it has started. Sorry, Reid, your shoes are going to stay wet for now. Any future technology here to stop rain would require major scientific breakthroughs, for example. We have to think, you know, from the ethical side, environmental side, geopolitical considerations as well is there. How do you feel about the rain. Do you enjoy the rain or do you dread it? Of course, in this country where we do not have a lot of rain, we look at it in a very positive way. Maybe others look at it in terms of, you know, floods and negatively, but from our side, because we have this water scarcity, we do not have this much of rain as other countries.

23:47So we look at it very positively. You know, it gives us this happiness feel when we receive the rain. So it seems that people in monsoon-soaked India and the dry and arid UAE have something in common, at least. Most of us look forward to the rain. But how about in Europe? That's a very good question. Depends very much on the conditions. You know, vacation in the tropics, fantastic. I hear you. Going to work in November in Geneva, well, I'm not a big fan. I've lived in Europe and America, and I know that the rain in Switzerland, where Professor Jean-Pierre Wolfe lives and works, is probably a little chillier than the Mumbai monsoon.

24:32So my name is Jean-Pierre Wolfe, and I'm working at the physics department of the University of Geneva. And I've been working with the lasers for many years, especially for applications in the atmosphere and for climate. If we're going to find a way to stop the rain, it sounds like all the technology we currently have at hand just isn't going to cut it. We need something a little more sci-fi, like lasers. We've been thinking about using lasers to locally produce either clouds or rain. So basically condensation of water vapor in the air. Despite all the assurances from cloud seeding organizations around the world, I keep coming across concerns about its potential environmental impact.

25:22Although seeding chemicals such as silver iodide are used in minuscule quantities, could they gradually accumulate over time or interact with the microorganisms that live in the soil or the water? If you think about heavy metals like silver, it's definitely not good for biodiversity because silver is used to kill some insects. And this is ground soil pollution. Yeah, and we don't want that. That's right. So our approach was to replace the chemical by laser light, which will not produce pollution, neither in the water nor in the soil. Okay, so can you break that down for me? You're using lasers, which are narrow, high-energy beams of light.

26:05Yes. What exactly are they doing? Can you break it down for me? Sure, sure. So the lasers we use are a little bit particular. The sand flashes with extremely short duration. And this has an effect that during this very short duration, the peak power is very high. The power of the laser is so high that you can rip off an electron out of an atom or a molecule and produce an ion. Now stick with me here because there's a lot of physics involved. Instead of seeding the air with new particles, Jean-Pierre uses his high-powered laser to target particles that are already in the atmosphere. And anything is fair game.

26:52It can be dust, it can be hydrocarbon, small particles like sulfates and nitrates, even nitrogen molecules, oxygen molecules. We can ionize these guys as well. His laser is powerful enough to change the molecular structure of the particle to rip off an electron, which leaves the particle with a negative electrical charge. This is what's called an ion. And the clever thing about that is that charged particles are always hydrophilic. They attract water. So the condensation is more efficient on these particles than it would have been if they were neutral. Can you tell me what these experiments look like in practice?