[MUSIC] According to legend, when Kublai Khan’s army set sail to conquer Japan, a powerful typhoon, a Pacific tropical cyclone, arrived without warning, destroying Khan’s fleet and saving Japan.

When the Mongols returned a few years later with an even bigger army, another typhoon showed up and turned the invading fleet into driftwood.

Thinking their emperor had summoned the storms to rescue them, the Japanese named these typhoons “divine wind”, or kamikaze.

Native cultures in the Caribbean and Central America, like the Taíno and Maya, told stories of Huracán, a god who brought wind and storms.

Their ancient artifacts tell us they’d deduced these storms’ spiral shapes long ago, something Western scientists wouldn’t figure out until 1831.

Tropical cyclones are called by different names in different oceans, but Atlantic hurricanes, like Katrina and Sandy, are probably the most famous.

These storms are some of the nastiest monsters that Earth’s Gamemakers throw our way.

A fully formed hurricane can reach more than 100 miles across and travel hundreds of miles, and inside the eye, walls of clouds and ice can extend up through the entire lower layer of our atmosphere.

But how does a storm that big even happen?!

The basic ingredients for a hurricane are pretty simple: heat and wind.

The wind is easy.

Wherever we find low air pressure, air wants to move from the outside in to fill that low pressure.

This is what creates a hurricane’s super-strong winds.

But it doesn’t explain why they spin.

A hurricane’s winds want to move in a straight line, but their path is deflected thanks to the Coriolis effect.

If Earth wasn’t spinning, hurricanes wouldn’t either.

A 100 mph wind blowing north near the equator is also moving east, along with the direction of Earth’s rotation.

Because Earth is basically a sphere, the closer we get to the poles, the slower the rotation.

So as that wind moves north, the atmosphere and the Earth below it aren’t spinning as fast, and the sideways motion of the wind outruns Earth’s rotation, pushing it east.

The same thing happens for the winds on the other side, except they’re moving slower, so they get pulled to the west.

This is why cyclones spin clockwise in the southern hemisphere, and counterclockwise in the northern hemisphere.

Or anticlockwise.

Or whatever.

It’s also why hurricanes can’t form too close to equator, there isn’t enough difference in rotation at low latitudes to start them spinning.

Moving in closer to the eye, hurricane winds increase in speed, because of angular momentum, just like how a figure skater spins faster when they pull their arms in.

The second key ingredient is heat from warm water.

A hurricane works a lot like an engine.

Not the engine in your car, but an ideal engine, like the one developed by French physicist Nicolas Carnot.

First, a piston rises.

Normally, reducing the pressure cools a gas, but in a Carnot Engine, heat is added so the temperature remains constant even as the pressure drops.

But what happens if we turn off the heat, and keep dropping the pressure?

The gas cools!

Next, let’s reverse the first step, lowering the piston.

We’d expect this higher pressure to heat up the gas, but in our engine we’ve got a device to draw heat out so the temperature remains the same.

Finally, we keep compressing the piston, but we stop taking away heat, so the temperature rises along with pressure.

Ideal engines like this don’t exist in the real world, but a hurricane is one of the closest things we’ve got.

As winds flow towards the center of a hurricane, we’d expect the low pressure to cool them, but the air is kept warm thanks to massive amounts of water evaporating.

And I mean massive!

An average hurricane can carry more than a hundred billion pounds of water!

So how does evaporation keep wind warm?

Changing water from liquid to vapor, to rip those molecules apart and let them fly free, takes energy.

This is why you feel cool when you get out of the pool, it’s what happens to a bowl of soup that’s left out too long.

Evaporating water takes heat with it.

That heat is stored in the wind, so the temperature stays the same even as the pressure goes down, it’s just like the first step of our ideal engine.

Then as the air rises, our heat source is gone, so water vapor condenses into liquid again, releasing its stored heat into the atmosphere.

The temperature’s actually higher on top of a hurricane, thanks just to the heat released by condensing water vapor.

High up, the air releases radiation into space as it flows out and.

down, just like step 3.

Finally, that air sinks back to Earth, compressing and getting warmer, and the hurricane engine is ready for another cycle.

This feedback loop turns hurricanes into self-sufficient engines of destruction.

A typical tropical cyclone consumes the same amount of power as the entire United States.

As long as there’s enough warm water, it keeps feeding itself, getting larger and larger until… Well, is there a limit to how powerful a hurricane can be?

Size and rainfall are the biggest predictors for how bad a storm will be, but hurricane intensity is rated by wind speed.

Category 1 are the weakest, while anything over 157 mph is lumped in Category 5.

Using equations for Carnot’s ideal engine and data about Earth’s climate, scientists say 190 mph should be the maximum theoretical windspeed.

But after Typhoon Haiyan hit 195 mph in 2013, does that mean it’s time to add a Category 6?

Let’s get real.

After a certain wind speed, all destruction pretty much looks the same, so it doesn’t make much sense to add another category of “bad”.

But it gives us something to think about.

As Earth’s climate warms, we’re putting more heat, more fuel into these engines of destruction, and it's another reason to take our foot off the gas pedal.

Stay curious.

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There’s a huge spinning storm in Jupiter’s southern hemisphere, the giant red spot.

Jupiter spins in the same direction as Earth, but the red spot storm spins counterclockwise, the opposite direction of southern hemisphere hurricanes.

Why is that?

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