Where Do Hurricanes Form? What You Need to Know

Most people know that hurricanes are incredibly dangerous storms that can cause deadly consequences and devastating damage. But it’s not until hurricane season arrives that people begin thinking about how hurricanes come about and where they form. Ahead, get all the details you need to know.

Where do hurricanes form? And how does a hurricane begin?

Hurricane Florence

Hurricane Florence | NASA via Getty Images

Vox reports that a hurricane in the Atlantic Ocean typically begins as a “tropical disturbance,” or “organized thunderstorm activity that stretches at least 100 miles across and maintains its identity for more than 24 hours.” During the summer, these disturbances often begin as storms moving westward off the coast of Africa in “tropical waves.”

The National Ocean Service characterizes a tropical wave as a low-pressure area that “moves through the moisture-rich tropics, possibly enhancing shower and thunderstorm activity.” If meteorologists spot a tropical disturbance that they think could develop into something more, they designate it an “investigative area.”

What comes next for the tropical disturbance?

Under the right conditions, a tropical disturbance can begin spinning around a low-pressure center, Vox reports. When the storm starts spinning, meteorologists classify it as a “tropical cyclone” or “tropical depression.” But this only happens when the conditions are just right. The water needs to be warm enough (80 degrees Fahrenheit or warmer). Plus, there has to be enough moisture in the lower and middle part of the atmosphere. And local winds have to follow a pattern that enables the depression to spin.

LiveScience reports that all low-pressure systems, including hurricanes, rotate counterclockwise in the northern hemisphere. That’s because when an area of low atmospheric pressure forms, winds begin to flow toward the center of it. Meanwhile, the Earth rotates under the atmosphere. In the northern hemisphere, the planet’s spin “causes an apparent deflection of the wind to the right, as seen from above.” That effect forces a counterclockwise rotation for the storm.

Then what happens to the tropical cyclone?

Vox reports that the next step for the tropical cyclone — which can become a tropical storm — requires the correct pressure conditions. When the pressure in the center of a tropical cyclone drops, air rushes in and creates strong winds. The air rushing in rises and cools. As LiveScience reports, “The ocean feeds warmth and moisture into the developing storm, providing energy that causes the warm air in the center to rise faster. It condenses high in the atmosphere, creating thunderstorms.”

If the system strengthens to the point that wind speeds surpass 39 mph, meteorologists designate it a “tropical storm.” They also give the storm a name. Vox notes that it’s the U.S. National Hurricane Center that officially makes the call on when a tropical depression or cyclone becomes a tropical storm. Researchers at the center use data from islands, buoys, and reconnaissance aircraft to make that call.

Is this when it becomes a hurricane?

If a tropical storm passes over a region of particularly warm water and doesn’t encounter much wind shear — conditions that only happen in the late summer months in the Atlantic — it can intensify quickly, Vox reports. “As that happens, the pressure in the center drops even further, and the winds really pick up. The system gets rounder and often forms a clearly defined ‘eye.'”

When the winds reach sustained speeds of 74 mph or more, meteorologists classify the storm as a hurricane. It then gets categorized by the Saffir-Simpson Scale based on wind speed and the kind of damage it can cause.

What happens next?

Hurricanes can prove deadly when they make landfall. But Vox notes that they can also weaken to tropical storms again as they move over land or cooler water, and don’t have as much warm, moist area to fuel them. When wind speeds drop below 75 mph, the hurricane becomes a tropical storm.

It can later degrade further into a post-tropical cyclone. But even tropical cyclones that aren’t technically hurricanes can inflict devastating damage with torrential rain, dangerous surf, beach erosion, high winds, and widespread flooding.

Read more: Why Some People Don’t Leave Before a Hurricane

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