Picture a spinning storm so wide it could cover the whole state of Texas. That's a hurricane. It starts as nothing but warm ocean water and grows into the biggest storm on Earth. Let's find out how.
What Exactly Is a Hurricane?
A hurricane is a giant spinning storm that forms over warm ocean water. Imagine a pinwheel of clouds, rain, and wind. The small ones are about 100 miles (160 km) wide. The biggest ones stretch more than 1,000 miles (1,600 km) across. That's wider than Texas!
But not every ocean storm is a hurricane. To earn that name, a storm's winds have to reach at least 74 mph (119 km/h).1 That's fast enough to snap tree branches and peel shingles off a roof. A storm goes through two smaller stages first.
Tropical Depression
First comes the tropical depression. It's a swirl of thunderstorms with winds under 39 mph (63 km/h). Think of it as a baby storm just getting started. Most of them fizzle out and never grow up.
Tropical Storm
If the winds pick up to between 39 and 73 mph (63 to 118 km/h), it becomes a tropical storm. This is when it gets a name, like Maria or Alex. Tropical storms bring heavy rain and gusty wind. They're not as strong as a full hurricane, but you still take them seriously.
Hurricane
Once the winds hit 74 mph (119 km/h) or higher, it's officially a hurricane.1 Now the storm is well organized, with a clear spin you can see all the way from space.
Here's something neat: this same storm has different names around the world. Near Japan and the Philippines, people call it a typhoon. Near India and Australia, they call it a cyclone. Same storm, different name, kind of like how some people say "soda" and others say "pop."
How Does a Hurricane Form?
A hurricane doesn't just appear. It needs a few things to come together, almost like a recipe. Miss one ingredient and the storm never forms. Here's what it takes.
Ingredient 1: Warm Ocean Water
The most important ingredient is warm ocean water. The sea surface has to be at least 80°F (about 26.5°C), and that warmth has to reach deep, down to about 165 feet (50 m).2 Why so deep? Because a storm churns the ocean like a giant spoon. If only the top is warm, the storm stirs up cold water and cools itself off. Deep warm water is like a big battery that keeps the hurricane powered up.
Ingredient 2: Rising, Wet Air
Warm water evaporates fast. That means the water turns into an invisible gas called water vapor, and it floats up into the sky. As that warm, wet air rises, it cools. The vapor turns back into tiny droplets and builds tall thunderstorm clouds. Making those clouds releases heat, and the heat pushes even more air up. It's a chain reaction: warm water makes vapor, vapor makes clouds, clouds make heat, and heat lifts more air.
Ingredient 3: The Coriolis Effect (Earth's Spin)
Here's the really cool part. As air rushes toward the center of the growing storm, Earth's spin makes that air curve instead of going straight. Scientists call this the Coriolis effect. North of the equator, where the United States is, it makes hurricanes spin counterclockwise. Picture water swirling down a giant drain, except it's wind swirling around a storm.
Right at the equator, the Coriolis effect is too weak. That's why hurricanes form at least 5 degrees of latitude away from it, roughly 345 miles (about 550 km) north or south.
Ingredient 4: Calm Winds Up High
Sometimes winds high in the sky blow a different way than the winds down low. That's called wind shear, and it's bad news for a young storm. Strong shear is like a hand shoving over a tower of blocks. It rips the storm apart before it can organize. A hurricane needs the winds at every height to blow in roughly the same direction. Then the storm can stack up tall and grow.
Ingredient 5: Room to Spin
Because a hurricane needs the Coriolis effect, it needs some distance from the equator to get going. So most hurricanes form between 5 and 30 degrees of latitude, north and south.2 That's why the warm, tropical parts of the Atlantic, Pacific, and Indian Oceans see the most storms.
The Parts of a Hurricane
Look at a hurricane from space and you'll see a huge, beautiful spiral of clouds. But it isn't one big blob of bad weather. It has parts, and each part has a job.
The Eye
Right in the center is a calm spot called the eye. It's usually 20 to 40 miles (30 to 65 km) wide. Stand in the eye and you might see blue sky and feel only a light breeze. It can feel strangely peaceful.
Don't be fooled, though. The eye is wrapped in the most dangerous part of the whole storm. Every year, people think the hurricane is over when the calm eye arrives, step outside, and get caught when the other side comes roaring back. We study these storms for a living, and that trick still fools people every season. So if the wind ever goes quiet during a hurricane, that's the moment to stay inside, not go out.
The Eyewall
The ring of clouds wrapped around the eye is the eyewall. This is where the storm is fiercest. The strongest winds, the heaviest rain, and the tallest thunderstorms are all packed in here. If the hurricane were a boxing glove, the eyewall would be the fist. When a storm reaches land, the spot the eyewall crosses takes the worst hit.
The Rainbands
Curling out from the eyewall like the arms of a pinwheel are the rainbands. These are long ribbons of rain and thunderstorms. They can reach hundreds of miles from the center. So even if you're far from the eye, a rainband can still bring pouring rain, gusty wind, and sometimes even a small tornado. The rainbands are what make a hurricane so wide.
The Stronger Side
Here's a surprise: one side of a hurricane is usually stronger than the other. North of the equator, the right side is the tougher one, if you're looking the way the storm is moving. That's because the storm's own forward motion adds to the wind on that side. Say a hurricane has 100 mph (160 km/h) winds and is rolling forward at 15 mph (24 km/h). The right side might feel 115 mph (184 km/h), while the left side feels closer to 85 mph (136 km/h).
Measuring a Hurricane's Strength
Scientists rate a hurricane's strength with the Saffir-Simpson Hurricane Wind Scale. It runs from Category 1, the weakest, up to Category 5, the strongest.3 Here's how it breaks down.
Category 1: 74–95 mph (119–153 km/h)
A Category 1 hurricane can damage roofs, snap big tree branches, and knock down power lines. It's the weakest kind of hurricane, but it's still a real storm that deserves respect.
Category 2: 96–110 mph (154–177 km/h)
A Category 2 does a lot more. It can rip off roofs, pull up shallow-rooted trees, and block roads with debris. The power can stay out for days.
Category 3: 111–129 mph (178–208 km/h)
At Category 3, a storm becomes a "major hurricane." It can wreck mobile homes, blow out high-rise windows, and badly damage sturdy houses. Neighborhoods can be hard to live in for a while afterward.
Category 4: 130–156 mph (209–251 km/h)
A Category 4 causes terrible damage. It can tear off roofs and walls, topple big trees, and cut a town off by wrecking its roads and power lines.
Category 5: 157+ mph (252+ km/h)
Category 5 is the top of the scale, the strongest kind of hurricane there is. The good news is they're rare. Only a handful have ever reached the United States coast at full Category 5 strength.
Remember: Categories 3, 4, and 5 are all "major hurricanes." Those are the ones forecasters watch most closely.
Here's the honest part, and it's important. Even with all our satellites and supercomputers, we still can't say exactly how strong a hurricane will get before it lands. We track where a storm is going pretty well. Guessing its final strength is much harder. That's why grown-ups prepare for a storm one category stronger than the forecast, just in case.
When Is Hurricane Season?
Hurricanes don't happen all year. They have a season, like baseball or soccer. In the Atlantic Ocean, along the east coast of the United States, hurricane season runs from June 1 through November 30. That's six months! But the storms aren't spread evenly across it.
The busiest stretch is from mid-August through mid-October. That's when the ocean is warmest and the sky is friendliest for building storms. If a big hurricane is in the news, there's a good chance it's during this peak.
The eastern Pacific season starts a bit earlier, on May 15. Those storms usually stay out over open water, though they can reach Mexico and Central America.
How Many Storms in a Year?
In a normal Atlantic season, you can expect about 14 named storms. About 7 of those become hurricanes. And about 3 of the hurricanes become major hurricanes, Category 3 or higher. (Those numbers come from NOAA's 30-year average, 1991 to 2020.1) Some years are quiet. Others are wild, with 20 or more named storms.
Cool Hurricane Facts
Hurricanes aren't only scary. They're also amazing. Here are a few facts to share with your friends.
A Hurricane Is an Energy Giant
As a hurricane builds its clouds and rain, it releases a staggering amount of energy. NOAA figured out that the energy is about 200 times all the electricity made by every power plant on Earth, combined.2 The wind is the part you can feel, but the real engine is the heat set loose when water vapor turns into clouds and rain.
How Hurricanes Get Their Names
Every hurricane gets a name from a list made ahead of time, and the lists repeat every six years. If a storm is especially destructive, its name is retired and never used again, out of respect. Katrina, Sandy, Harvey, and Maria were all retired for that reason.
Where the Word "Hurricane" Comes From
The word "hurricane" comes from Hurakan, the Mayan god of storms, wind, and fire. The Maya of Central America knew these storms long before there were weather forecasts. Spanish sailors picked up the word, and it eventually became "hurricane" in English.
The One Thing to Remember
A hurricane runs on warm ocean water. As long as it stays over warm sea and the winds up high stay calm, it can keep growing. Move it over land or cold water and it loses its fuel and fades. That's the whole secret of a hurricane in one sentence.
Want to see what it's like to be inside one of these storms, safely? Head over to Hurricane Safety for Kids to learn your job before, during, and after a storm. Or find out how scientists study hurricanes with flying planes and space satellites.
Sources
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National Hurricane Center. Tropical Cyclone Climatology. NOAA. https://www.nhc.noaa.gov/climo/ ↩ ↩2 ↩3
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Hurricane Research Division. Frequently Asked Questions. NOAA Atlantic Oceanographic and Meteorological Laboratory. https://www.aoml.noaa.gov/hrd-faq/ ↩ ↩2 ↩3
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National Weather Service. Saffir-Simpson Hurricane Wind Scale. NOAA. https://www.weather.gov/mfl/saffirsimpson ↩