Take a look at this map.

Which of these thousands of tropical cyclones is not like the others?

For some reason, hurricanes are terrified of South America, except one.

According to conventional wisdom, Hurricane Catarina shouldn't have happened.

So when I learned how it did, pretty much everything I thought I knew about hurricanes went right out the window.

And Catarina isn't the only weird thing on this map.

In fact, the longer I looked at it, the less sense it made.

So we're going on a tour of some of the most interesting anomalies here because it turns out that they're the best places to learn about how hurricanes actually work.

And by the end, we'll understand what made the least likely storm in history possible.

And we'll see how this map is changing, raising the stakes for millions who live in their path, including what might be the riskiest country on earth.

All right, so before we get into it, let's get our terminology straight.

Every storm on this map is what we call a tropical cyclone, a rotating low pressure storm system that forms over warm tropical water.

And they're the most powerful storms on earth.

Once sustained winds reach 39 miles per hour, they get an official name.

And at 74 miles per hour, they become hurricane, typhoon, or simply a cyclone, depending on where you live in the world.

Different names, but the same kind of storm.

This map of every tropical cyclone since 1985 is more than just beautiful.

It also hides secrets about how the world's most powerful storms work.

And surprisingly, I think it might actually make the most sense to start where there aren't any cyclones at all.

As long as we've been keeping track, not one tropical cyclone has ever crossed the equator.

That's because one crucial ingredient in the recipe doesn't exist here, a phenomenon called the Coriolis effect.

Imagine you're riding on a carousel and you wanted to shoot an arrow at a target on the opposite end of the carousel.

You might aim right at the target, but you'd actually miss because the target would move out of the way.

From above, the arrow would travel in a straight line.

But from your perspective, spinning on the carousel, it would veer off to the right.

That's the coriolis effect, and it works the same way on earth because the planet is rotating.

Air traveling long distances across its surface curves the same way.

To the right in the northern hemisphere and to the left in the southern hemisphere.

And it's this curvature that determines which way cyclones spin, counterclockwise in the north and clockwise in the south.

But the strength of the coriolis effect is not the same everywhere.

Its strongest near the poles weakens as you approach the equator and drops all the way to zero directly on top of it.

And without enough of that planetary vorticity or turning effect, thunderstorms can't organize into a stable rotating vortex.

It turns out you don't have to be very far off the equator.

Generally speaking, you have to be, I, I would say more than five or six or seven degrees to make it easy for a hurricane to form.

Once you're forward of that, the rotation is not a limiting factor on whether you have a hurricane.

And to Kerry's point, just a few hundred miles north of the equator over the Western Pacific is where you find the brightest, most densely packed blob of storm tracks on the entire planet.

Sure enough, when you look at the numbers, this basin produces far more tropical cyclones than anywhere else on earth, and more of our major cyclones too, meaning category three or higher.

So what's so special about it?

First, this is where you find the appropriately named warm pool, which is Earth's largest reservoir of warm ocean water.

And warm water is hurricane fuel.

But the warm pool isn't just large, it's also deep.

Warm water is especially deep, but that doesn't really help the hurricane form, but it helps it get to its high intensity.

Hurricanes are so vigorous, right, that they create upwelling, which can bring up colder water.

In most places that upwelling can weaken a storm by cutting off its supply of heat.

But here, the warm pool extends so far below the surface that there isn't much cold water to pull in the first place, allowing the storm to continue to intensify.

And because of this nearly bottomless supply of fuel, hurricanes can sometimes reach what scientists call their maximum potential intensity, a theoretical upper limit of hurricane strength, which in the Western North Pacific is near 200 miles per hour.

And we do observe on rare occasions hurricanes that get right up to this limit.

The most extreme example of this to date was probably Super Typhoon Haiyan in 2013.

Haiyan made landfall in the Philippines with sustained winds of 196 miles per hour and became the deadliest typhoon in the country's recorded history, killing over 6,000 and leaving another four million homeless or displaced.

So yeah, this area is clearly ground zero for tropical cyclones, and nowhere feels the effects more than the Philippines.

An average of eight to nine tropical cyclones make landfall in the Philippines each year.

And largely because of this insane exposure.

It's actually ranked number one on the world risk index or disaster risk.

And the danger is magnified by the country's geography.

The Philippines is made up of thousands of islands.

So despite only being about the size of Arizona, it has nearly doubled the amount of coastline of the entire United States, which means millions of people live in low-lying coastal communities, vulnerable to destructive winds and storm surge.

And inland, its steep mountainous terrain is especially prone to flash floods and landslides.

And if all of this wasn't enough, the Philippines also sits directly on the Pacific Ring of Fire, exposing it to major earthquakes and volcanic eruptions.

Earthquakes, tsunamis, floods, cyclones, droughts, and sea level rise.

The most riskiest country on earth is the Philippines.

If the vast supply of warm ocean water helps make the Western North Pacific the most active hurricane region on Earth, what makes the tropics around South America?

Well, the complete opposite.

Until Catarina in 2004, nobody living on the continent had ever experienced a hurricane.

Along the Pacific Coast, the explanation is pretty straightforward.

The Humboldt or Peru current carries cold water northward, while powerful coastal upwelling pulls even colder water up from below.

Together, they create some of the coldest tropical waters on earth, making tropical cyclone formation virtually impossible here.

But what about the Atlantic side?

While the water on this side is warmer, it's still not quite warm enough.

Ocean circulation carries heat from the South Atlantic into the North Atlantic, cooling the south and warming up the north.

That extra warmth helps fuel hurricanes in the North Atlantic.

But in the South, the loss of that heat keeps potential intensity so low that tropical cyclones basically can't form here.

And storms here face another obstacle.

Vertical wind shear simply means that the wind is changing speed and/or direction with height.

That can essentially pull a storm apart, tilting its entire structure and disrupting the flow of heat and moisture that keeps the engine running.

But there's one storm that miraculously beat the odds here.

It was so unlikely it was dubbed the impossible storm.

Over 93% of all tropical cyclones form over water warmer than 80 degrees Fahrenheit or 26.5 degrees Celsius.

But in mid-March of 2004, Catarina strengthened over water between just 24 and 25 degrees Celsius, an extraordinarily rare occurrence.

So how was that possible?

Well, it all comes down to how hurricanes work, a lot like the engine in your car.

A Hurricane is a really beautiful example of a heat engine.

The heat engine is just an engine that converts heat energy into some other form.

Basically, it works by moving heat from a warm place to a colder one and converting some of that energy into work.

In a car engine, that work turns the wheels.

The case of a hurricane is wind.

The ocean is the warm side of that engine.

It pumps heat and moisture into the storm.

And the cold side is at the top of the troposphere, which is the layer of the atmosphere where pretty much all of our weather happens.

Now that place of hurricane is way, way up in the tropical atmosphere.

It's 10, 11, 12 miles up where the temperatures are some of the coldest temperatures you can find anywhere in our planet, including Antarctica.

And this is the counterintuitive reason the tropics are actually so perfect for hurricanes.

It's not just that the ocean is really warm here.

It's actually because the tropics is where the troposphere is the thickest, which also means the tropics is where the troposphere gets coldest at the top.

And the greater the temperature difference between the sea surface and the top of a hurricane, the more efficiently the storm can convert heat into wind.

And that same physics is also what makes polar hurricanes possible, which are actually a thing.

And when that difference is large enough, even if it's at high latitudes, you can have a hurricane.

Which brings us back to Catarina.

Even though it didn't form in polar waters, the ocean temperature was extremely cool for a hurricane.

But the temperature difference between the ocean and the atmosphere above was still large enough to power Catarina's heat engine.

What made that possible was an extremely rare atmospheric blocking pattern.

It brought in unusually cold air above Catarina while also splitting up upper level winds around the storm, creating a zone of weak wind shear.

That rare combination gave Catarina both the temperature difference to power it and the calm atmosphere it needed to organize.

And as you might imagine, Brazil was not prepared.

One Brazilian meteorologist even told Reuters "Brazil has never had a hurricane and it's not having one now."

Fortunately, regional meteorologists in Santa Catarina came to the opposite conclusion, issuing warnings and essentially inventing a hurricane playbook on the fly, adapting information from FEMA and other US resources.

Catarina may have been an extraordinary outlier, but because of that, it's also an extraordinarily helpful illustration of how hurricanes work.

And all of the ingredients we've talked about, ocean and atmospheric temperatures, wind shear, humidity, and global wind patterns can be fed into increasingly sophisticated climate models.

And as those models have gotten better at actually simulating hurricanes, scientists have gotten a much clearer picture of how these storms could change as the planet warms.

And for hurricane prone regions everywhere, from the Philippines to the US Gulf and East Coast, that matters because preparing for what comes next means remembering one more thing.

This map isn't fixed.

Some models predict tropical cyclone activity expanding further towards the polls, potentially exposing communities at higher latitudes that historically haven't dealt with hurricanes as often.

Anyways, like I said, there's a lot to unpack in this map, but I hope I answered all your questions about hurricanes.

Or if you still have a few, please let us know in the comments.

As always, thanks so much for watching and we'll see you next time on Weathered.