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Warm-Up
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Which stage happens first in the life cycle
of a middle latitude cyclone?
What type of front forms when the surface
position of the front does not move?
What happens when an active cold front
overtakes a warm front?
Severe Storms
Chapter 20, Section 3
Thunderstorms
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Thunderstorm – a storm that generates lightning
and thunder; frequently produce gusty winds, heavy
rain, and hail
Thunderstorms may be produced by a single
cumulonimbus cloud, or it may be associated with
clusters of cumulonimbus clouds along a cold front
At any given time, there are an estimated 2000
thunderstorms in progress on Earth (~45,000 a day
and 16 million a year); the U.S. experiences ~100,000
a year
Thunderstorms form when warm, humid air rises in an
unstable environment
The life span of a single cumulonimbus cell within a
thunderstorm is only about an hour or two, but as the
storm moves, it is constantly getting fresh supplies of
warm, humid air
U.S. Distribution of Thunderstorms
Stages in the Development of a
Thunderstorm
Tornadoes
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Tornadoes – violent windstorms that take the form
of a rotating column of air called a vortex; the vortex
extends downward from a cumulonimbus cloud
In the U.S., ~770 tornadoes are reported each year;
greatest occurrence between April and June
Most tornadoes form in association with severe
thunderstorms
An important process in the formation of many
tornadoes is the development of a mesocyclone (see
diagram)
Pressures within some tornadoes can be as much as
10% lower than the surrounding area, causing air
near the ground to be “sucked” into the vortex
One scale used to measure tornado intensity is the
Fujita tornado intensity scale
Tornado Suction Vortices
Formation of a Mesocyclone
U.S. Tornado Incidences
Hurricanes
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Hurricanes – whirling tropical cyclones that produce
winds of at least 119 kilometers per hour (also known
as typhoons, cyclones, and tropical cyclones)
At sea, they can generate 15-meter high waves
capable of destruction hundreds of kilometers away
A hurricane is a heat engine that is fueled by the
energy given off when huge quantities of water vapor
condense
Hurricanes develop most often in the late summer
when water temperatures are warm enough to
provide the necessary heat and moisture to the air
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Eye Wall – doughnut-shaped wall that
surrounds the center of the storm, where the
greatest wind speeds and heaviest rainfall
occur
Eye – center of the storm where precipitation
ceases and winds subside
The intensity of a hurricane is described using
the Saffir-Simpson scale
Storm Surge – a dome of water about 65 to
80 km wide that sweeps across the coast
where a hurricane’s eye moves onto land
A hurricane weakens when it moves over cooler
ocean water and land
Hurricane Cross-Section
Hurricane Katrina
Assignment
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Read Chapter 20, Section 3 (pg. 571-577)
Do Section 20.3 Assessment #1-8 (pg. 577)
Study for Chapter 20 Quiz!