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What Are Hurricanes?
September 3, 2014

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Hurricanes are large, swirling storms. They produce winds of 119 kilometers per hour (74 mph) or higher. That's faster than a cheetah, the fastest animal on land. Winds from a hurricane can damage buildings and trees.

Hurricanes form over warm ocean waters. Sometimes they strike land. When a hurricane reaches land, it pushes a wall of ocean water ashore. This wall of water is called a storm surge. Heavy rain and storm surge from a hurricane can cause flooding.

Once a hurricane forms, weather forecasters predict its path. They also predict how strong it will get. This information helps people get ready for the storm.

There are five types, or categories, of hurricanes. The scale of categories is called the Saffir-Simpson Hurricane Scale. The categories are based on wind speed.

Category 1: Winds 119-153 km/hr (74-95 mph) - faster than a cheetah

Category 2: Winds 154-177 km/hr (96-110 mph) - as fast or faster than a baseball pitcher's fastball

Category 3: Winds 178-208 km/hr (111-129 mph) - similar, or close, to the serving speed of many professional tennis players

Category 4: Winds 209-251 km/hr (130-156 mph) - faster than the world's fastest rollercoaster

Category 5: Winds more than 252 km/hr (157 mph) - similar, or close, to the speed of some high-speed trains
 

What Are the Parts of a Hurricane?
Eye: The eye is the "hole" at the center of the storm. Winds are light in this area. Skies are partly cloudy, and sometimes even clear.

Eye wall: The eye wall is a ring of thunderstorms. These storms swirl around the eye. The wall is where winds are strongest and rain is heaviest.
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Rain bands: Bands of clouds and rain go far out from a hurricane's eye wall. These bands stretch for hundreds of miles. They contain thunderstorms and sometimes tornadoes.

How Does a Storm Become a Hurricane?
A hurricane starts out as a tropical disturbance. This is an area over warm ocean waters where rain clouds are building.

A tropical disturbance sometimes grows into a tropical depression. This is an area of rotating thunderstorms with winds of 62 km/hr (38 mph) or less.

A tropical depression becomes a tropical storm if its winds reach 63 km/hr (39 mph).

A tropical storm becomes a hurricane if its winds reach 119 km/hr (74 mph).

What Makes Hurricanes Form?
Scientists don't know exactly why or how a hurricane forms. But they do know that two main ingredients are needed.

One ingredient is warm water. Warm ocean waters provide the energy a storm needs to become a hurricane. Usually, the surface water temperature must be 26 degrees Celsius (79 degrees Fahrenheit) or higher for a hurricane to form.

The other ingredient is winds that don't change much in speed or direction as they go up in the sky. Winds that change a lot with height can rip storms apart.

How Are Hurricanes Named?
There can be more than one hurricane at a time. This is one reason hurricanes are named. Names make it easier to keep track of and talk about storms.

A storm is given a name if it becomes a tropical storm. That name stays with the storm if it goes on to become a hurricane. (Tropical disturbances and depressions don't have names.)
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Each year, tropical storms are named in alphabetical order. The names come from a list of names for that year. There are six lists of names. Lists are reused every six years. If a storm does a lot of damage, its name is sometimes taken off the list. It is then replaced by a new name that starts with the same letter.

You can see the lists of names here.

How Does NASA Study Hurricanes?
NASA satellites take pictures of hurricanes from space. These pictures are shown on TV. They are also shown on the Internet.

Some satellite instruments measure cloud and ocean temperatures. Others measure the height of clouds and how fast rain is falling. Still others measure the speed and direction of winds.

NASA scientists use data, or facts, from satellites and other sources to learn more about hurricanes. The data helps them understand how hurricanes form and get stronger. The data also helps forecasters predict the path and strength of hurricanes.

Did you know that dust storms from Africa might affect hurricanes? Two NASA satellites have a tool that helps scientists study how dust impacts hurricanes.

NASA also flies airplanes into and above hurricanes. The instruments onboard gather details about the storm. Some parts of a hurricane are too dangerous for people to fly into. To study these parts, NASA uses airplanes that operate without people.

NASA also carries out special projects to learn more about hurricanes. These projects use a mix of instruments on satellites, on aircraft and on the ground.

More About Hurricanes
› The Space Place: How Do Hurricanes Form?   →


Read What Are Hurricanes? Grades 5-8

Return to Homework Topics Grades K-4

Dan Stillman, Institute for Global Environmental Strategies

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A large, white swirling storm over the ocean
NASA studies hurricanes to learn how they form and to better predict where they will go.
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NASA
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The large swirling eye of Hurricane Dean
The eye of Hurricane Dean is shown as it moved through the Caribbean.
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NASA
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A view of the Caribbean and the Gulf of Mexico in August 2005 showing warmer surface water temperatures in yellow and orange
Warm water in the Gulf of Mexico and the Caribbean during the late summer months helps to fuel hurricanes.
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NASA
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Page Last Updated: September 3rd, 2014
Page Editor: NASA Education