0:00The Engine
This is Hurricane Isabel, in the Atlantic, in September 2003: a Category 5 storm, an engine the size of a country, turning. If you follow its spiral bands, you'll see that the winds near the surface all turn the same way, counter-clockwise. Every hurricane north of the equator turns like this, and every one south of it turns the other way. That pattern isn't chance. The planet itself creates it.
0:27In this episode, we'll find out what a hurricane runs on, why it spins the way it does, and what happens inside the eye. We'll go back to two storms that changed America, and see how forecasts that once missed by more than 350 miles now miss by about 100.
0:47We'll start with the fuel. A hurricane is a heat engine, and it runs on warm water. The sea surface has to be at least 26.5 °C, about 80 °F, and the water has to stay that warm to a depth of some 50 meters. This map shows the sea in late August 2005, with the warmest water in red. It also explains why the season peaks at the end of summer, when the ocean has had months to absorb the Sun's heat.
1:14The engine works like this. Warm water evaporates, and the moist air above it rises. As the air climbs, its water vapor condenses into cloud, and condensation releases heat. That heat makes the air rise even faster, which draws in more moist air from the sea, so the cycle keeps feeding itself. In a single day, an average hurricane releases more than 100 times as much heat as all the electricity the world generates.
1:45Why It Spins
Next comes the spin. Air rushes in toward the low pressure at the center of the storm from every side, and on a planet that stood still, it would flow straight in. But the Earth is turning, and anything that travels a long way across its surface is deflected, to the right in the Northern Hemisphere and to the left in the Southern. That deflection is called the Coriolis effect.
2:10As a result, the air never reaches the center directly. It curves around it, and the whole storm winds itself into a spiral, counter-clockwise in the north and clockwise in the south. Once you know that, any satellite picture of a hurricane tells you which hemisphere it is in.
2:28Right at the equator, however, there is no sideways deflection at all. Within about 5 degrees of it, some 550 kilometers on either side, the Coriolis effect is too weak to start a storm turning. That is why hurricanes almost never form there, and almost never cross from one hemisphere into the other.
Now let's look into the center of a storm. This is Hurricane Patricia, off the coast of Mexico, in October 2015. Its winds reached 345 kilometers an hour, or 215 miles an hour, the strongest ever measured in a hurricane. At its center, there is a hole in the clouds: the eye.
3:11Here is the eye of Isabel, up close. Inside it the air is sinking, so the wind is light and the sky can even clear. The swirls you can see are mesovortices, smaller whirlpools of air turning within the eye itself. An eye like this is usually 30 to 60 kilometers across, and around it stands the eyewall, a ring of towering cloud where the winds blow hardest. When the eye passes over a town, the wind suddenly drops, and then it returns just as hard from the opposite direction.
3:43The lowest air pressure ever measured at sea level was recorded inside an eye. Typhoon Tip, in the western Pacific in 1979, bottomed out at 870 millibars. Tip was also the largest tropical cyclone on record, about 2,200 kilometers across. Laid over North America, it would cover half the width of the United States.
4:10The Names
Why do storms have names at all? Forecasters were often tracking several storms at once, and long descriptions were easily confused over the radio. Atlantic storms were first named in 1950, using the radio alphabet of the time: Able, Baker, Charlie. Women's names were introduced in 1953, and men's names joined the lists in 1979.
4:33Names make a real difference when several storms are active at once. On September 14, 2020, five tropical cyclones were spinning in the Atlantic at the same time, something seen only once before, in 1971. The World Meteorological Organization keeps six lists of names for the Atlantic, used in rotation, so each name comes back every six years. The exception is a storm so deadly or so costly that its name is retired for good, as happened with Katrina, Maria, Ian and Helene.
5:061900 and 2005
The first of those two storms struck on September 8, 1900. Galveston, Texas, was then one of the richest cities in the state, built on a sand island whose highest point stood less than 9 feet above the sea. There were no satellites and no radar, and ships at sea had no radio, so nobody could see how strong the storm had grown out in the Gulf.
5:28The sea rose more than 15 feet and swept across the whole island. By morning, about 8,000 people were dead, and it remains the deadliest natural disaster in American history. Galveston rebuilt behind a seawall 17 feet high, and then raised much of the city, house by house, on jacks.
5:49A hundred and five years later came Katrina. On August 28, 2005, it was a Category 5 storm in the middle of the Gulf, with winds of 175 miles an hour, and the whole coast from Louisiana to Florida lay in its path.
6:06Katrina came ashore in Louisiana the next morning, weaker by then, as a Category 3. Much of New Orleans, however, sits below sea level behind levees, and the storm surge broke through them in about 50 places. Eighty percent of the city flooded. Nearly 1,400 people died, and the damage came to $125 billion.
6:30Most of that water did not fall from the sky. It poured in from the lake and the canals when the flood walls gave way. Since then, the city has built a $14 billion ring of levees, gates and pumps, one of the largest civil works projects the Army Corps of Engineers has ever built.
6:52The Forecast
Modern forecasters can see these storms coming far better than anyone could in Galveston. Every forecast now comes with a cone, like this one. The cone does not show the size of the storm. It shows where the center will probably go, and two times out of three, the center stays inside it.
7:09In 1990, the National Hurricane Center's three day forecasts missed the storm's position by more than 350 miles, on average. By the mid 2020s, the average miss was about 100 miles. That improvement came from satellites, from hurricane hunter aircraft that fly straight through the eyewall, and from computer models running on some of the fastest machines on Earth.
7:33What remains hard to predict is strength. Over very warm water, some storms strengthen astonishingly fast, a process forecasters call rapid intensification, defined as winds rising by at least 35 miles an hour within a single day. In October 2024, Hurricane Milton went from Category 1 to Category 5 in about a day, on its way to Florida.
8:00Still Spinning
Now look at the other side of the world. This is Cyclone Winston in the South Pacific, in February 2016, two days before it struck Fiji as one of the strongest storms ever recorded in the Southern Hemisphere. It runs on the same warm water and the same engine, but look at its spiral. This one turns clockwise.
8:22So, three things to remember. First, a hurricane is a heat engine that runs on warm sea water, at least 26.5 °C. Second, the Earth's rotation gives it its spin, counter-clockwise north of the equator and clockwise south of it, which is why hurricanes almost never form at the equator itself. And third, forecasts of where a storm will go have improved more than threefold since 1990, but how strong it will become is still the hard part.
8:38The next time you see a hurricane from space, look at the way it turns. If it turns counter-clockwise, it is north of the equator, and if it turns clockwise, it is south of it. And remember what it runs on: the warm sea beneath it, turned into wind by a planet that never stops turning.
