Hurricanes
Dana Desonie, Ph.D.
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Printed: July 25, 2012
AUTHORS
Dana Desonie, Ph.D.
www.ck12.org
C ONCEPT
Concept 1. Hurricanes
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Hurricanes
• Explain how and where hurricanes form.
• Describe how hurricanes are measured and the damage that they can cause.
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
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.
Formation
Hurricanes arise in the tropical latitudes (between 10o and 25o N) in summer and autumn when sea surface temperature are 28o C (82o F) 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 28o C (82o F), 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.
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FIGURE 1.1
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 1.1).
TABLE 1.1: Saffir-Simpson Hurricane Scale
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Category
1 (weak)
Kph
119-153
Mph
74-95
2 (moderate)
154-177
96-110
Estimated Damage
Above normal; no real
damage to structures
Some roofing, door, and
window damage, considerable damage to vegetation, mobile homes, and
piers
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Concept 1. Hurricanes
TABLE 1.1: (continued)
Category
3 (strong)
Kph
178-209
Mph
111-130
4 (very strong)
210-251
131-156
5 (devastating)
>251
>156
Estimated Damage
Some buildings damaged;
mobile homes destroyed
Complete roof failure on
small residences; major
erosion of beach areas;
major damage to lower
floors of structures near
shore
Complete roof failure on
many residences and industrial buildings; some
complete building failures
Damage
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 1.2).
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FIGURE 1.2
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/flashfl
ood.swf.
An animation of a radar image of Hurricane Katrina making landfall is seen here: http://upload.wikimedia.org/wiki
pedia/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.nat
ionalgeographic.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.at
mos.uiuc.edu
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.
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Concept 1. Hurricanes
Summary
• 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.
Practice
Use this resource to answer the questions that follow.
MEDIA
Click image to the left for more content.
1. What are hurricanes?
2. Where do most hurricanes begin to form?
3. What three components are required for a hurricane to form?
4. What direction do hurricanes spin?
5. What is the eye?
6. What are typhoons and cyclones?
Review
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?
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