- Does it feel like cities have been getting more and more dangerous?

Well, they have, but it's not for the reasons many think.

Back in 2002, researchers noticed something very strange in the Houston area.

It looked like there was more lightning activity over and downwind of the city.

It was as if the city was attracting storms or even creating them.

But what was going on in Houston that could change the weather so dramatically?

We know that rain, flooding and heat waves have become more extreme in our warming world, but we wanted to know, is it possible that we've been doing a giant weather modification experiment without even knowing it?

- When you really think about rain and clouds, the clouds are at a height, which is way, way, way about the city.

And the city in comparison, are tiny dwarves.

They are the small blobs.

So there are instances where we have seen the storm actually knows there is a city down there.

And the question is how?

- Today more than half of the world lives in urban areas and major cities around the world are projected to grow in the future.

So if we're doing something to make them more dangerous, it's important that we understand how.

But this got me to wonder if cities can accidentally make weather worse, can they intentionally make it better?

- Are we coming to an era where we are going to look for climate controlled cities?

- Two decades ago when researchers noticed unusual lightning over Houston, they reasoned that rainfall might be higher there as well.

They then used satellite precipitation monitoring from a new NASA mission and found a 44% increase in rainfall inside and downwind of the city.

But new research shows that just 150 miles away, Austin, Texas seems to be repelling storms getting less rain than its rural surroundings.

So what's the difference?

First off, this research was done by one of my favorite climate scientists, Dr. Marshall Shepherd, who I talked to about which cities are most at risk as the climate warms in my new series Weathered Earth's Extremes.

Today researchers have figured out why cities can repel or attract storms, and in an era of increasingly extreme drought and rain, the stakes are high for modifying precipitation where the majority of people live.

But to understand what's going on here, we first need to look at a totally different weather phenomenon.

Heat.

We know that cities can impact temperature.

It's called the urban heat island effect.

Let's quickly look at how it works.

First, cities use a lot of materials like concrete and brick that absorb and retain heat.

Second, the lack of vegetation means less shade and less cooling through evapotranspiration and third tall buildings act as street canyons trapping the heat emitted from human activities like transportation and air conditioning.

This all means that large cities can be as much as 10 degrees Celsius warmer than their rural surroundings.

This year, Phoenix, Arizona was off the charts hot.

The city saw temperatures of over 100 degrees or higher for a record of 113 consecutive days, and they spent a total of 70 days above 110 degrees.

But just outside the city in a suburban development called Robson Ranch, only 50 days reached 110 degrees.

That's 28% fewer days.

And as global warming continues to disproportionately warm our cities, the people who live in them are at growing risk.

But what does urban heat island have to do with rain?

Well, that heat doesn't stay on the ground - In some instances because the city itself is hot.

You start getting the air about the city becoming hotter.

When you are generally higher temperature, your pressure goes down - And this localized area of low pressure can act like a magnet attracting cooler air from nearby rural regions.

- So you now have a region which is slightly lower pressure, and so the wind starts coming towards the city - As the air heated by the city rises drawing in cooler air, it creates upward motion or convection high into the atmosphere.

The rising air cools causing moisture to condense into clouds, resulting in stronger, more concentrated storms and heavier rain.

Researchers observed the same thing in other cities, but were they isolated cases or part of a larger pattern where cities attract and create storms?

As global satellite data became more robust, researchers were finally able to look for a larger pattern.

- We can use this data and observed global cities and also compare with their nearby rural rainfall to see if there is any urban difference show up from this result.

- After analyzing rainfall data for over 1000 cities worldwide, over a 20 year period, a clear trend began to emerge.

63% of global cities had more rainfall than their rural surroundings, and the larger the city, the larger the effect.

So how significant is it?

They found an approximate 10% increase in summer afternoon rainfall on average in large US cities.

But that 10% hides some of the almost unimaginable extremes, which we'll look at in a bit.

They also found that this effect is growing.

- We separate the cities who are showing this urban rain island effects and generally check the average magnitude of this rainfall anomaly.

We find this number is increasing, it's almost doubled in the 20 years, - And it turns out that increasing urban heat alone doesn't explain the full extent of this effect.

So what else is going on?

Well, the answers are kind of surprising.

- So in cities we have these tall buildings, this increases the roughness.

- That's right.

Cities are rough, at least as far as storms are concerned.

This basically means that the buildings create friction slowing down storms as they pass over.

Instead of gliding smoothly, the storms are disrupted by the tall structures causing them to linger over the city, which gives them more time to dump more rain.

And then there's one final ingredient and this goes out to all the conspiracy theorists.

- And this is like you know, cloud seeding to some extent, and that is pollution in the city.

The cars that we use, the the particles start going into the sky and then they start interacting with the clouds.

And because of that, now the clouds that are coming there, they're juicy with all the moisture.

And if you have the right instability, they're either going to precipitate over the city or they're going to get all that energy and then move downwind of the city and precipitate there.

- One of the most dramatic examples of the urban rain island effect occurred in 2017 when Hurricane Harvey, a storm labeled as a once in a 2000 year event dropped in unprecedented 40 to 60 inches of rain in Houston over just four days.

A 2018 study ran two different simulations of Hurricane Harvey hitting the city, one with Houston's urbanized landscape and the other replacing the city with crop land, the results were striking.

Urbanization made the extreme flooding event 21 times more likely.

The sheer amount of rain that fell multiplied by the urban rain island effect was clearly one of the main factors that made Hurricane Harvey so devastating for Houston, but it wasn't the only one.

Houston's rapid urban expansion has led to a significant increase in impervious surfaces such as concrete and asphalt, which prevent water from being absorbed into the ground.

This left much of Hurricane Harvey's rainfall with nowhere to go, resulting in catastrophic flooding.

Economic damages were estimated at around $125 billion, making it one of the costliest natural disasters in US history.

This issue with impervious surfaces and flooding is a problem in cities all across the country.

And then when you factor in the increasing strength of storms due to climate change, we begin to see how vulnerable cities really are to rain and flooding events.

- And as we go into the future, we are probably going to have storms which are more juicier because they're going to have more moisture because they're warmer.

And now you have an instability that is coming from the city.

That's like your one two punch you're giving that storm and making it precipitate.

That essentially is what is happening in terms of how cities are changing rainfall city by city globally.

- So cities on average are amplifying the impacts of climate change when it comes to heat, rain and flooding.

But in the beginning of the episode we mentioned that Austin is actually seeing less rain - When you see these two cities, one large, one relatively emerging and they showing opposite signature in close vicinity.

So it is somewhat similar air mass that is coming its way that shows the power of the local topography in the city in terms of how it is churning the environment above it and how that can change the rains associated with that.

So that was fascinating - And it turns out this mostly comes down to an unexpected detail about Austin, its shape.

- If I have a city like Austin in a topography and it is a trapezoidal shape where one part of the city is much longer than the other, then the storms actually go away from the city.

- The exact reasons why a city's topography and shape affect rainfall in this way are still an area of active research, but the implications of this anti rain island effect could be useful for helping make our cities more resilient to climate change - Rather than waiting for global treaties for all the world's nations and leaders to agree on what is a path ahead.

Cities themselves can have a master plan, how they grow, what is going to be their footprint, and perhaps that can change the temperature as well as the rainfall.

And that is powerful.

- To truly reduce the risk in cities and beyond, we have to reach net zero carbon emissions as soon as possible.

But in the meantime, we can meaningfully tackle some of the problem at a local level, which I find quite inspiring.

We can reduce impervious surfaces to lower flood risk and cool the environment.

We can rethink future development in cities to make them less rough for passing storms.

And maybe most importantly, we can drastically reduce air pollution that helps seed and strengthen storms.

- I'm not saying putting more trees or planning a city differently will stop a hurricane.

What I do want to highlight is how we design our city, whether we do vertical growth, whether we do sprawl, how do we use cooling, whether it is engineering technology or by putting trees.

All of that could have an implication in whether the environment will create more rain, less rain, whether the hurricane will concentrate the rain in certain pockets or it'll spread the rain.

And when we are looking for against something where we feel completely helpless, we should try it out.