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This is Hurricane Isabel,
out in the Atlantic, in September 2003:

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a Category 5 storm, an engine
the size of a country, turning.

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Follow the spiral bands
and you'll see that

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the winds near the surface all turn the

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same way, counter-clockwise.

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Every hurricane north
of the equator turns like this,

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and every one south of it
turns the other way.

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Nobody planned that.

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The planet does it.

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In this episode, we'll find
out what a hurricane runs on,

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why it spins the way it does,
and what's going on inside the eye.

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We'll go back to the two
storms that changed America,

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and see how forecasts that once missed by
more than 350 miles now miss by about 90.

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Let's start with the fuel.

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A hurricane is a heat engine,
and it runs on warm water.

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The sea has to be at least
26.5 °C, that's about 80 °F,

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and it has to stay that
warm some 50 meters down.

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This map shows the sea in late August
2005, with the warmest water in red.

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That's why the season
peaks at the end of summer,

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when the ocean has had all
summer to soak up the sun.

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Here's how the engine works.

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Warm water evaporates,
and the moist air rises.

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As it climbs, the vapor condenses into
cloud, and condensing gives off heat.

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That heat makes the air rise even faster,

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which pulls in more moist air from the
sea, so the loop keeps feeding itself.

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In a single day,

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an average hurricane releases more
than 100 times as much heat as all the

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electricity the world generates.

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Now, the spin.

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Air rushes in toward the low pressure at
the center of the storm from every side.

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On a planet that stood still,
it would flow straight in.

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But the Earth is turning,

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and anything that travels a long
way across it gets bent off course,

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to the right in the Northern Hemisphere
and to the left in the Southern.

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That bending is called
the Coriolis effect.

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So the air never reaches
the center directly.

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It curves around it, and the whole
storm winds itself into a spiral,

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counter-clockwise in the north
and clockwise in the south.

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Once you know that,

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any satellite picture of a hurricane
tells you which hemisphere it's in.

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Right at the equator, though,
there's no sideways push at all.

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Within about 5 degrees of it,
some 550 kilometers on either side,

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the Coriolis effect is too
weak to get a storm turning.

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That's why hurricanes
almost never form there,

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and why they never cross from one
hemisphere into the other.

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Now let's look into the middle.

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This is Hurricane Patricia,
off the coast of Mexico, in October 2015.

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Its winds reached 345 kilometers
an hour, that's 215 miles an hour,

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the strongest ever
measured in a hurricane.

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And right in the center,
there's a hole in the clouds.

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That's the eye.

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Here's the eye of Isabel, up close.

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Inside it the air is sinking, so the wind
is light and the sky can even clear,

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and those swirls you can see are smaller
whirlpools of air turning within the eye

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itself.

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An eye like this is usually
30 to 60 kilometers across.

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Around it stands the eyewall,

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a ring of towering cloud
where the winds blow hardest.

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When the eye passes over a town,
the wind suddenly drops,

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and then it comes back just
as hard from the other direction.

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The lowest air pressure ever measured
at sea level was inside an eye.

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Typhoon Tip, in the western Pacific,
in 1979, bottomed out at 870 millibars.

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Tip was also the largest tropical cyclone
on record, about 2,200 kilometers across.

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Laid over North America, it would cover
half the width of the United States.

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So why do storms have names?

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Because forecasters were often
tracking several at once,

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and long descriptions got
mixed up over the radio.

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Atlantic storms were first named in 1950,
from the radio alphabet: Able, Baker,

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Charlie.

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Women's names came in 1953,
and men's names joined the lists in 1979.

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Here's why that matters.

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On September 14, 2020,

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five tropical cyclones were spinning
in the Atlantic at the same time,

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something seen only once before, in 1971.

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The World Meteorological Organization
keeps six lists of names for the Atlantic,

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used in turn, so a name
comes back every 6 years,

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unless a storm was so deadly or so costly
that it's retired for good, like Katrina,

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Maria, Ian and Helene.

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September 8, 1900.

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Galveston, Texas, was one
of the richest cities in the state,

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built on a sand island whose highest point
stood less than 9 feet above the sea.

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There were no satellites and no radar,
and ships at sea had no radio,

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so nobody could see how strong
the storm had grown out in the Gulf.

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The sea rose more than 15 feet
and swept across the whole island.

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By morning, about 8,000 people were dead.

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It's still the deadliest natural
disaster in American history.

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Galveston rebuilt behind a seawall 17 feet
high, and then raised much of the city,

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house by house, on jacks.

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A hundred and five years
later came Katrina.

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On August 28, 2005, it was a Category
5 in the middle of the Gulf,

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with winds of 175 miles an hour,

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and the whole coast from Louisiana
to Florida in its path.

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Katrina came ashore in Louisiana the next
morning, weaker by then, a Category 3.

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But much of New Orleans sits
below sea level, behind levees,

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and the storm surge broke through
them in more than 50 places.

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Eighty percent of the city flooded.

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Nearly 1,400 people died,
and the damage came to $125 billion.

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Most of that water
didn't fall from the sky.

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It poured in from the lake and the canals
when the flood walls gave way.

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Since then, the city has built a $14
billion ring of levees, gates and pumps,

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the biggest civil works project the Army
Corps of Engineers has ever taken on.

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So how well can we see them coming?

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Far better than Galveston could.

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Every forecast now comes
with a cone, like this one.

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The cone isn't the size of the storm.

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It shows where the center will probably
go, and two times out of three,

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the center stays inside it.

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In 1990,

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the Hurricane Center's three day forecast
missed the storm's position by more than

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350 miles, on average.

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By the mid 2020s,
it was missing by about 90.

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That's thanks to satellites,

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to hurricane hunter aircraft that
fly straight through the eyewall,

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and to computer models running on some
of the fastest machines on Earth.

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What's still hard to predict is strength.

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Over very warm water, some storms
strengthen astonishingly fast.

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Forecasters call it rapid intensification,

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when the winds climb by at least
35 miles an hour in a single day.

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In October 2024,
Hurricane Milton went from

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Category 1 to Category 5 in about a day,

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on its way to Florida.

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Now look at the other side of the world.

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This is Cyclone Winston in the South
Pacific, in February 2016,

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2 days before it struck Fiji as one of
the strongest storms ever recorded in the

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Southern Hemisphere.

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It runs on the same warm
water and the same engine.

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But look at the spiral.

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This one turns clockwise.

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So the next time you see a hurricane
from space, look at the way it turns.

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If it's counter-clockwise, it's north
of the equator, and if it's clockwise,

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it's south.

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And remember what it runs on:
the warm sea beneath it,

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turned into wind by a planet
that never stops turning.

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Thanks for watching Earth Explained.

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