What Causes Hurricanes?

Learn about these violent storms and why they’re dangerous.

WATCH: Hurricanes are massive storms with deadly force. Find out how they form, and what's being done to better predict their impact.

Every year, coastal regions brace themselves for violent windstorms known as hurricanes. But how do these storms form and grow?

The oversimplified answer: Warm ocean water plus the Earth’s eastward rotation.

“They’re heat engines,” said meteorologist Jeff Masters of the website Weather Underground in a previous interview. “They take heat from the oceans and convert it to the energy of their winds. They’re taking thermal energy and making mechanical energy out of it.”

The natural engine that is a hurricane is fueled by warm, moist air. The storms move heat from the ocean surface high into Earth’s atmosphere. They can travel thousands of miles from the tropics toward the Earth’s poles.

According to NOAA’s National Hurricane Center, the average hurricane eye—the still center where pressure is lowest and air temperature is highest—stretches 20 to 30 miles across, with some even growing as large as 120 miles wide.

The strongest storms, equivalent to Category 5 on the Saffir-Simpson scale, have sustained winds that exceed 155 miles an hour.

Why Are Hurricanes Dangerous?

While hurricanes are categorized based on their wind speeds, wind isn’t typically the most dangerous part of such storms. “It’s the storm surge,” said Kerry Emanuel, an atmospheric scientist at MIT, in an earlier interview. The storm surge is the bulge of water built up in front of a cyclone or hurricane courtesy of its winds.

It’s the number one killer in hurricanes, Emanuel explained. “That’s what killed people in Katrina, it’s what killed people in Sandy and in Haiyan.” (Read “Charts Show How Hurricane Katrina Changed New Orleans.”)

Emanuel likened a storm surge to a tsunami. One just happens to be caused by earthquakes (tsunamis), while the other is generated by hurricanes.

Flash flooding caused by intense rains is also a major killer, Emanuel said. “Hurricane Mitch [in 1998] killed 12,000 people and it was all from flash flooding.”

Then comes wind that blows around debris. Hurricane Andrew in 1992 is an example of this. “It didn't really cause too much of a storm surge,” the atmospheric scientist said, “but boy did it blow a lot of buildings down.”

Climate change will likely increase the frequency of “the high-end hurricanes,” Emanuel said.

And those powerful storms have the potential to produce a lot of rain, flooding, and strong storm surges.

Is That a Hurricane or a Cyclone?

What’s the difference between hurricanes, cyclones, and typhoons? Actually, they’re all the same weather phenomenon. Scientists just call these storms different things depending on where they occur.

In the Atlantic and northern Pacific, the storms are called hurricanes, after the Caribbean god of evil, named Hurrican.

In the northwestern Pacific, the same powerful storms are called typhoons. In the southeastern Indian Ocean and southwestern Pacific, they are called severe tropical cyclones.

In the northern Indian Ocean, they're called severe cyclonic storms. In the southwestern Indian Ocean, they're just called tropical cyclones.

To be classified as a hurricane, typhoon, or cyclone, a storm must reach wind speeds of at least 74 miles an hour.

If a hurricane’s winds reach speeds of 111 miles an hour, it is upgraded to an “intense hurricane.”

If a typhoon hits 150 miles an hour—as Usagi did in 2013—then it becomes a “supertyphoon.”

While the Atlantic hurricane season runs from June 1 through November 30, the typhoon and cyclone seasons follow slightly different patterns.

In the northeastern Pacific, the official season runs from May 15 to November 30. In the northwestern Pacific, typhoons are most common from late June through December. And the northern Indian Ocean sees cyclones from April to December.

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