What causes hurricanes?

We recognize the howling sound of the wind and the pounding of rain during a hurricane. During the worst of them, we panic over flooding and other damage. There is no doubt that hurricanes can be a devastating force of nature. What many may have lingering questions on, however, is where these windy, wet storms come from.

While it may not help in preparing for the next big hurricane, understanding Mother Nature and how hurricanes form is just another step toward better understanding our world.

What is a hurricane?

Aerial photo of a hurricane approaching Florida
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Before we get to the “how,” it’s important you understand the “what.” Chances are you already know hurricanes are powerful storms accompanied by extremely high winds and rain, but that could also be describing typhoons and cyclonic storms.

A hurricane is a weather system that forms over the Atlantic Ocean or eastern Pacific, just near the Earth’s equator. They are especially dangerous to the southern and eastern United States, Mexico, the Atlantic island nations, and the very northern tip of South America.

Similar storms that form near Asian and South African waters are just as powerful but known by different names. While the name may change, the “how” remains the same.

The early stages of a hurricane

Tropical road with heavy rain
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There is a reason why hurricanes form only over warm water. Think of a hurricane as a car. There is a key component required to keep it moving — gasoline. Without petrol, your car won’t even start. The same concept is true for hurricanes, except the catalyst is warm, moist air.

At the start of a hurricane, the warm air rises away from the surface, creating a pocket of low air pressure below. The low pressure attracts high-pressure air, which then becomes moist and warm and rises. The cycle repeats over and over, all the while the warm air that’s rising is cooling and forming a key component in the hurricane that’s forming.

Physical signs of a pending hurricane

Dark clouds over a road
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While that cycling of low and high-pressure air is unobservable by the human eye, there is something happening that can be seen. When the rising warm air starts to cool, it forms cumulonimbus clouds, or a low-lying, vertical cloud that’s common during thunderstorms. Within the newly formed cloud system, wind spins and continues to increase in speed. Much like with our car, the warmth from the ocean and evaporating water from its surface act as fuel, causing stronger winds and more clouds.

At the center of this windy, cloudy system is the eye of the storm, the calmest part of the storm where low air pressure gathers and high air pressure filters in from above. Though the system that’s formed over the ocean is the start of a hurricane, there is a classification scale that it must reach before officially earning the title.

Classifying a hurricane

Clouds forming the shape of a hurricane
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Before a storm system can be considered a hurricane, it must reach beyond winds of 74 miles per hour (mph). At the start of the system’s formation, it’s just a normal storm that may bring heavy rains and the occasional clap of thunder. When those winds reach 39 mph, the system is then categorized as a tropical storm. While they can cause minimal flooding, tropical storms are generally not dangerous.

When winds reach speeds of 74 to 95 mph, the storm is labeled a Category 1 hurricane. Slightly more substantial damage is expected when winds reach 96 to 110 mph, or a Category 2 hurricane, with extensive damage occurring at 111 to 129 mph, or Category 3. Evacuations are likely by the time a hurricane hits a Category 4 (130 to 156 mph) or a Category 5 (157 mph or higher).

According to the Atlantic Oceanographic & Meteorological Laboratory, since the first recordings of a hurricane in 1851, there has been just over 1,500 tropical storms. From them, 915 hurricanes have formed, but since storms lose power the closer they get to land, only 294 have made landfall.

Preparing for a hurricane

Disaster preparation list and supplies
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Knowing what a hurricane is may feed your curiosity, but understanding how to best prepare for one will keep you and your loved ones safe. When hurricane season approaches, it’s important to have an evacuation plan, a supply of non-perishable goods, a stock of batteries and lanterns, a portable weather radio, basic tools, drinking water, and prescription medication.

If you live in an area prone to hurricanes, cut down weak trees or branches, consider installing hurricane doors and storm shutters, and seal any outside openings.