[♪ INTRO].

Humans have been trying to control  the weather since ancient times. But even with modern technology, we can barely even predict the  weather, let alone manipulate it.

At least, on purpose, because it  turns out there are quite a few ways that humans do influence the weather,  and not just through climate change. Even on local levels, human  activity can produce more rain. Most of the time, it is by accident, but purposely increasing rainfall might  not be as far-fetched as it sounds, either.

Some of the biggest markers  of human activity are cities, where people have drastically  altered the landscape to fit lots of living and  working spaces all in one place. And research has shown that,  compared to the areas around them, cities get more than their fair share of rain. One big reason for that is urban heat.

Cities tend to be a few degrees  hotter than their surroundings, thanks to a number of factors. For one, they’re full of heat-absorbing  materials like concrete, asphalt, and steel. Then there’s all the heat-producing machines like cars, air conditioners, and  heaters all stuffed into a small area.

And since dry, flat surfaces  like rooftops and roads have replaced a lot of the natural vegetation,  there isn’t as much shade or natural cooling. So you get what’s called an urban heat island. And thanks to all that heat, water  molecules on the ground have more energy, so they can more easily break the hydrogen bonds that hold them in their liquid  form and turn into vapor.

As a result, there’s extra water vapor  in the air over these urban heat islands. And that sets off the process of forming rain. Because rain forms as water vapor in the  atmosphere condenses and forms little droplets.

Once enough water droplets have come together, they get heavy enough to fall out of the cloud. So, the more water vapor there is in  the air, the more easily that happens. As a result, in cities, and the areas around them,  rainfall is both more frequent and more intense.

Like, one study of cities around the  world found that, downwind of cities, the average rainfall was as much as fifty  percent greater than it was upwind of cities. And a recent study in the eastern Mediterranean found that for each degree  Celsius the temperature rises, there’s around a 3% increase  in the intensity of rainfall. But urban heat isn’t the only reason  cities produce excess rainfall.

The jagged landscape full of skyscrapers  and other buildings also plays a role. That’s because, as air flows into a city, it slows down as it encounters tall  buildings and gets trapped in place. So as more air sweeps across the land or  water and comes to a halt over the city, it creates a kind of pileup called convergence.

As the relatively moist air hanging  over the ground gets forced upward, it delivers extra water vapor to the clouds. Once again, as that water condenses into droplets, it can eventually fall back down as rain. And there’s another way cities  can create convergence, too.

In some cases, they can slice storms in  half, sending clouds flowing around the city, a lot like a rock splits the  flow of water in a creek. And when the parts of the storm  recombine downwind of the city, the colliding air can get pushed up again, producing more rain clouds and  making the storm more intense. But cities don’t just make storms rainier.

They can also make storms longer. Thanks to all those tall buildings  creating friction with the air, storms that roll in and lose their  momentum can get stuck in the city. Researchers think that’s what happened when Hurricane Harvey struck Houston in  2017 and dropped rain on it for days.

Simulations have suggested that  friction from Houston’s buildings slowed down the storm enough to increase  rainfall by up to fifty percent. Now, it can be hard to say exactly  how much of a role humans play in specific rainfall events, because weather  is an unpredictable and complex thing. But by looking at broad weather patterns,  we can see our impact more clearly.

And in one incredible study,  researchers studied old weather patterns to show that humans have increased  rainfall by testing nuclear bombs. Scientists had suspected for a while that  nuclear bombs might influence rainfall because radioactive decay floods the  atmosphere with electric charges. And even though water molecules  don’t have any overall charge, there’s a slight difference in  electric charge across each molecule.

So, when there are other  charged particles in the air, the oppositely charged side of  the water gets attracted to them. Pairing up with these electric  charges basically accentuates water’s slight charge difference and  helps droplets stick together more easily. And just like in the first two examples,  the more droplets stick together, the heavier they get, making them  more likely to fall to the ground.

But in practice, scientists weren’t sure  how significant this effect would be. So authors of a 2020 study published  in the journal Physical Review Letters found a clever way to work it out,  by using historical weather data to investigate the impact of  nuclear bombs on rainfall levels. They gathered the data from a weather station  on a remote island in northern Scotland, where there was very little pollution  to interfere with their results.

First, they looked at data  documenting the amounts of various particles in the atmosphere  during the nuclear testing of the 1960s. The data included information about levels of the  long-lasting radioactive element strontium-90, which is produced in nuclear reactions. The researchers found that  those levels peaked around 1962, and the levels of charged ions  in the air peaked with them.

Both levels then dropped a few years  later, after a treaty was signed in 1963 that reduced the amount of above-ground testing. So then the team looked at historical  weather data and found that, before and after the treaty, rainfall and  cloud cover in the region also changed. In particular, during the  years of heavy nuclear testing, the cloud cover was thicker, and  the rainy days were extra rainy:.

They had about 24% more rainfall than usual. Now, these findings weren’t totally surprising, they supported scientists’ longstanding theory  about how electric charge influences rain. But what’s incredible is that this weather station was thousands of kilometers  away from nuclear tests.

In other words, bombs dropped  in Nevada and Kazakhstan had an impact on the weather  in Scotland for years. But it does not take nuclear fallout to  change the atmosphere in ways that make rain. Ordinary aerosols have been doing it for ages.

Now, an aerosol can be a lot of things. Technically, it refers to any  fine particles suspended in air, both natural and human-generated. It includes things like volcanic ash as  well as pollutants from cars and factories.

All of these things can act as a nucleus  for water vapor to condense around. Because when water condenses in  the atmosphere, it always does so around some kind of particle, like dust,  which is known as a nucleation site. So, when there are more particles in the air, water vapor condenses into a  bunch of different droplets.

If they’re light enough, the  droplets won’t always fall. Which is why, most of the time,  aerosols actually reduce rainfall. The water vapor gets spread out so  thinly among all these particles that the droplets that fall  from the cloud are really tiny, and they end up evaporating  before they reach the ground.

Plus, aerosols can darken the  sky and cool down the air, which produces the opposite  effect of the urban heat island. But researchers have proposed that, under the right conditions,  aerosols can increase rainfall, too. If a cloud has lots of water vapor to begin with, then there’s enough of it to go  around to all the nucleation sites.

In that case, the fact that  there are more possible nuclei just means more droplets can  get big enough to fall as rain. So rainfall may end up being more intense  than it would have been otherwise. In general, although humans  definitely do make it rain, it’s usually by accident, and  not really under our control.

But for our last example, we will look at one way people have been trying for decades  to make rain, or snow, on purpose. It’s called cloud seeding. The key to cloud seeding is the  fact that, like in the last example, water needs some kind of nucleus to latch onto  before it can form droplets or ice crystals.

And in the 1940s, a pair of scientists  wondered if they could get more water molecules to turn into snow and ice by adding lots  of extra nucleation sites to clouds. They tried it out in the lab, using silver  iodide to turn cold water vapor into snow, and they actually had some success. But it’s a hard idea to test in the real world.

Like, how do you know it wasn’t gonna rain anyway? Plus, you can’t exactly  repeat the experiment because weather is too weird to have the exact same  weather conditions on any two days ever. But the idea has still  attracted a lot of attention.

Various researchers have come up with  other materials that might act as nucleation sites for ice or rain, like dry ice  or salt, and others have even put it to the test. Like, in the 1960s, the U. S. government  actually tried to weaken tropical storms through cloud seeding in an  effort called Project Stormfury.

They thought that by seeding  clouds near the eye of a storm, they might be able to make the eye grow, which they hoped would slow  down wind speeds overall. That didn’t work, and later  research into hurricanes showed that the idea was flawed even in principle. But in 2018, scientists  finally made a breakthrough when they directly observed cloud  seeding in nature for the first time.

They sent two planes to inject  silver iodide into clouds over Idaho and used radar to watch the snow form. Unfortunately, even though this experiment  seemed to demonstrate true cloud seeding, we are still nowhere near any  kind of useful cloud control. We’re not gonna be able to  make rain over a cornfield or drain a cloud right before a sporting  event anytime in the near future.

There are just so many factors that  go into determining whether a cloud will unleash a torrent or a drizzle that  it’s hard to manipulate the process. But if there’s one thing to  take away from this video, it’s that humans can make huge impacts on the  planet we live on, for better or for worse. And the better we understand that power, the better decisions we can  make about how to use it.

Thanks for watching this episode of SciShow! And speaking of rain, if you’ve  ever wondered what it actually means when the forecast predicts a 50% chance of rain, we have an answer for you, and you  can catch our episode on that next. [♪ OUTRO].