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Warm Up
1) How does surface air flow in a middle-latitude cyclone in the
Northern Hemisphere?
a. convergent and counterclockwise
b. divergent and clockwise
c. convergent and clockwise
d. divergent and counterclockwise
2) Which of the following best describes the pressure in a
middle-latitude cyclone?
a. Pressure decreases toward the center.
b. Pressure increases toward the center.
c. Pressure remains the same everywhere.
d. The pressure is not predictable.
3) On a weather map, which type of front is shown by a line with
triangular points on one side?
a. cold
c. occluded
b. warm
d. stationary
Answers: 1) a. 2) a. 3) a.
Severe Storms
Chapter 20, Section 3
Thunderstorms
• 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
Concept Development
• Descirbe the stages in the development of a
thunderstorm.
• During the cumulus stage, strong updrafts
supply moist air that causes the cloud to grow
vertically. Usually within about an hour of the
initial updraft, heavy precipitation is released
from the cloud. Gusty winds, lightning, and
sometimes hail also are experienced during this
stage. Eventually, downdrafts dominate
throughout the cloud and the storm dies down.
Tornadoes
• 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
Concept Check
• How does a tornado form?
• Most tornadoes form in association with
severe thunderstorms.
Hurricanes
• 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
• 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
Concept Check
• How does a hurricane form?
• A hurricane develops when water
temperatures are warm enough to provide
the necessary heat and moisture to the air.
Assignment
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Read Chapter 20, Section 3 (pg. 571-577)
Do Ch. 20 Assessment #1-37 (pg. 583-584)
For Section 3: #’s 7-9, 16-20, 28-36
Study for Chapter 20 Quiz!