Learn what causes these deadly storms—and how to stay safe.

The wind at the beach is whipping at 95 miles an hour. Waves 16 feet tall are crashing down. Even sharks are heading out to calmer waters. A hurricane is on its way.

These powerful storms have different names depending on where in the world they form. They’re called hurricanes if they occur in the Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and eastern Pacific Ocean. In the western Pacific Ocean, they’re known as typhoons; in the southern Pacific and Indian Oceans, they’re called cyclones.

Scientists know them all as tropical cyclones. As many as 150 occur around the world each year.

How a hurricane forms

Hurricanes are strong storms that start in the ocean and have winds of at least 74 miles an hour. In the Northern Hemisphere (the part of Earth north of the equator), hurricanes generally occur between mid-August to late October. In the Southern Hemisphere (the area south of the equator), storm season is between April and December, with peak storm activity around May and November. During these times, oceans have warmer water, which is what a hurricane needs to form.

Hurricanes begin when a tropical depression forms in the ocean. A tropical depression is a line of rain showers and weak thunderstorms that circle around an area of low air pressure. If the water is at least 79°F, a hurricane might form.

The low air pressure causes the hot, humid air from the ocean to rise in a spiral shape. As that warm air rises, it releases heat, cools down, and condenses into gusty bands of clouds and storms. The low-pressure area continues to suck up hot, moist air, and the spiral gets stronger and faster.

When winds reach 39 miles an hour, the tropical depression becomes a tropical storm. When winds reach 74 miles an hour, it’s officially a hurricane.

As a hurricane moves over cooler water or hits land, it loses the warm water that fuels it and begins to weaken. But dangerous winds can still cause damage, and storm surges—when a strong storm pushes ocean water ashore—can flood coastal areas with more than 20 feet of water. Heavy rains and floods can continue far inland.

Understanding a hurricane’s strength

Hurricanes are classified by wind speed and the amount of damage that scientists predict the storm will cause when it reaches land. (In the United States, this is done by the National Hurricane Center, part of the National Oceanic and Atmospheric Administration.)

For example, a Category 1 storm has wind speeds of 74 to 95 miles an hour and will probably damage some homes and cause a few power outages. But a Category 5 hurricane is considered a powerful, destructive storm. These storms have winds of 156 miles an hour or more and will likely cause so much damage that people must abandon their communities, which will need months to clean up.

These rankings can help predict the severity of an approaching storm, but any hurricane can be devastating. For instance, Hurricane Katrina was only a Category 3 storm when it hit the U.S. Gulf Coast around Louisiana and Mississippi in 2005. The storm caused over $100 billion in damages, and nearly 2,000 people died.

Predicting the path

Meteorologists can predict the path that a hurricane will travel—and where it will make landfall—by analyzing changes in temperature, cloud formations, and air circulation patterns. That data is gathered by specialized aircraft, satellites, and weather surveillance radar from above the Earth. Other scientific devices that float in the ocean can measure winds, waves, and air and sea temperatures.

The information from these tools is used to create computer forecasting models used by scientists to learn when a storm is forming, where it might travel, and how severe it will be. That can help local officials release warnings or even evacuation orders in order to keep people safe from the coming hurricane.

Naming hurricanes

Once a storm’s wind speed reachers 39 miles an hour, it becomes a tropical storm and is given a name. The name is chosen from a list of about 20 names issued by the World Meteorological Organization at the beginning of the hurricane season. They alternate between female and male names.

But you won’t see David, Katrina, Andrew, or Laura on that list—those hurricanes were so destructive that the names have been removed. And you won’t hear of any hurricanes starting with Q, U, X, Y, or Z, since few names begin with those letters.

More powerful storms in the future?

In the future, scientists don’t expect to see more hurricanes, but they do expect more powerful ones. That’s because hurricanes get bigger more quickly in warmer water. As climate change causes ocean temperatures to rise, scientists predict that future hurricanes will be rainier and longer lasting.

How to survive a hurricane

The safest thing to do is to follow local instructions and leave the area, or evacuate, if officials say you should. Here are other tips for staying safe if a hurricane is headed your way.

Before a hurricane
• Know the evacuation routes in your neighborhood, and make sure your parents have put gas in the car.
• Check your yard to see if any branches are broken on trees and alert an adult if they are. (Those branches could break off and soar through a window!)
• Bring in anything that could blow away, like garbage cans and outdoor furniture.
• Close all your windows, storm shutters, blinds, and curtains.

During a hurricane
• If you haven’t evacuated, stay indoors where it’s safe.
• Fill all bathtubs and sinks with clean, cold water in case water lines are damaged or the supply becomes contaminated.
• Ride out the storm in an interior room, such as a hallway or closet on the lowest level of your house.
• Never go near a window, even if it’s boarded up.
• If it looks like the storm is going to hit your home, lie on the floor under a table or another sturdy object.
• Limit the use of cell phones to free up space for emergency calls.

After a hurricane
• Listen to weather reports to make sure the storm has passed. A calm sky might be just the hurricane’s eye, or center, and more storm fun will be on the way.
• If you evacuated, return only when officials say it’s safe.
• Check all food for spoilage. Remember: If in doubt, throw it out.

Adapted from the Nat Geo Kids book Extreme Weather by Thomas Kostigen, revised for digital by Laura Goertzel

Learn more about hurricanes, typhoons, and cyclones at National Geographic.