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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.
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