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9.26: Hurricanes

Difficulty Level: At Grade Created by: CK-12
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Why did New Orleans Mayor Ray Nagin call Hurricane Katrina "...a storm that most of us have long feared," as it approached New Orleans?

Hurricane Katrina nears its peak strength as it travels across the Gulf of Mexico. Hurricane Katrina was the most deadly and the most costly of the hurricanes that struck in the record-breaking 2005 season.


Hurricanes — called typhoons in the Pacific — are also cyclones. They are cyclones that form in the tropics and so they are also called tropical cyclones. By any name, they are the most damaging storms on Earth.


Hurricanes arise in the tropical latitudes (between 10o and 25oN) in summer and autumn when sea surface temperature are 28oC (82oF) or higher. The warm seas create a large humid air mass. The warm air rises and forms a low pressure cell, known as a tropical depression. Thunderstorms materialize around the tropical depression.

If the temperature reaches or exceeds 28oC (82oF), the air begins to rotate around the low pressure (counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere). As the air rises, water vapor condenses, releasing energy from latent heat. If wind shear is low, the storm builds into a hurricane within two to three days.

A cross-sectional view of a hurricane.

Hurricanes are huge and produce high winds. The exception is the relatively calm eye of the storm, where air is rising upward. Rainfall can be as high as 2.5 cm (1") per hour, resulting in about 20 billion metric tons of water released daily in a hurricane. The release of latent heat generates enormous amounts of energy, nearly the total annual electrical power consumption of the United States from one storm. Hurricanes can also generate tornadoes.

Hurricanes move with the prevailing winds. In the Northern Hemisphere, they originate in the trade winds and move to the west. When they reach the latitude of the westerlies, they switch direction and travel toward the north or northeast. Hurricanes may cover 800 km (500 miles) in one day.

Saffir-Simpson Scale

Hurricanes are assigned to categories based on their wind speed. The categories are listed on the Saffir-Simpson hurricane scale (Table below).

Saffir-Simpson Hurricane Scale
Category Kph Mph Estimated Damage
1 (weak) 119-153 74-95 Above normal; no real damage to structures
2 (moderate) 154-177 96-110 Some roofing, door, and window damage, considerable damage to vegetation, mobile homes, and piers
3 (strong) 178-209 111-130 Some buildings damaged; mobile homes destroyed
4 (very strong) 210-251 131-156 Complete roof failure on small residences; major erosion of beach areas; major damage to lower floors of structures near shore
5 (devastating) >251 >156 Complete roof failure on many residences and industrial buildings; some complete building failures


Damage from hurricanes comes from the high winds, rainfall, and storm surge. Storm surge occurs as the storm's low pressure center comes onto land, causing the sea level to rise unusually high. A storm surge is often made worse by the hurricane's high winds blowing seawater across the ocean onto the shoreline. Flooding can be devastating, especially along low-lying coastlines such as the Atlantic and Gulf Coasts. Hurricane Camille in 1969 had a 7.3 m (24 foot) storm surge that traveled 125 miles (200 km) inland.

The End

Hurricanes typically last for 5 to 10 days. The winds push them to the northwest and then to the northeast. Eventually a hurricane will end up over cooler water or land. At that time the hurricane’s latent heat source shut downs and the storm weakens. When a hurricane disintegrates, it is replaced with intense rains and tornadoes.

There are about 100 hurricanes around the world each year, plus many smaller tropical storms and tropical depressions. As people develop coastal regions, property damage from storms continues to rise. However, scientists are becoming better at predicting the paths of these storms and fatalities are decreasing. There is, however, one major exception to the previous statement: Hurricane Katrina.

Hurricane Katrina

The 2005 Atlantic hurricane season was the longest, costliest, and deadliest hurricane season so far. Total damage from all the storms together was estimated at more than $128 billion, with more than 2,280 deaths. Hurricane Katrina was both the most destructive hurricane and the most costly (Figure below).

Flooding in New Orleans after Hurricane Katrina caused the levees to break and water to pour through the city.

News about Hurricane Katrina from the New Orleans Times-Picayune: http://www.nola.com/katrina/graphics/flashflood.swf.

An animation of a radar image of Hurricane Katrina making landfall is seen here: http://upload.wikimedia.org/wikipedia/commons/9/97/Hurricane_Katrina_LA_landfall_radar.gif.

NASA’s short video, "In Katrina’s Wake": http://www.youtube.com/watch?v=HZjqvqaLltI.

Hurricanes are explored in a set of National Geographic videos found at National Geographic Video: http://video.nationalgeographic.com/video/environment/environment-natural-disasters/hurricanes. At this link, watch the following videos:

  • “Hurricanes 101” is an introduction to the topic.
  • “How Katrina Formed” looks at the history of Hurricane Katrina as it formed and passed through the Gulf coast.
  • Follow that up with “Doomed New Orleans,” which explores how the devastation to the city is a man-made disaster.
  • “The Hurricane Ike of 1900” looks at what happened in the days when there was little warning before a hurricane hit a coastal city.

Lots of information about hurricanes is found in this online guide from the University of Illinois: http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/hurr/home.rxml.


  • hurricane: Cyclone that forms in the tropics and spins around a low-pressure center.
  • tropical depression: A low pressure cell that rises in the tropics; thunderstorms arise here.


  • Hurricanes are actually tropical cyclones because they originate in the tropical latitudes.
  • The damage hurricanes cause is due largely to storm surge, but high wind speeds and rain also cause damage.
  • Hurricane Katrina was so damaging because the levees that protected New Orleans broke.


Use this resource to answer the questions that follow.


1. What is a tropical cyclone?

2. What add energy to the storm?

3. What is the eye?

4. What moves the storm?

5. What is the Saffir-Simpson Scale?

6. What are typhoons?

7. What causes a hurricane to lose steam?

8. What can storm surges cause?

9. Why is the study of hurricanes important?

10. What is the difference between a hurricane watch and a hurricane warning?


1. What is the difference between a hurricane and a mid-latitude cyclone?

2. How does a hurricane form? Where does the storm get its energy?

3. Under what circumstances does a hurricane die?




Cyclone that forms in the tropics and spins around a low-pressure center.
tropical depression

tropical depression

A low pressure cell that rises in the tropics; thunderstorms arise here.

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Difficulty Level:
At Grade
Date Created:
Feb 24, 2012
Last Modified:
Mar 23, 2016
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