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SCIENCE ~ CHAPTER 8
WEATHER
Miss Nelson
SECTION 4
Storms
ANTICIPATORY SET
What are the four types of fronts?
What happens when air masses with
different pressure and humidity meet?
STANDARDS
S 6.4.e –
Students know differences in pressure, heat, air
movement, and humidity result in change in
weather
THE BIG IDEA
What are the main kinds
of storms, and how do
they form?
KEY TERMS
 Storm – a violent disturbance in the
atmosphere
 Thunderstorm – a small storm with
heavy precipitation and frequent
thunder and lightning
 Lightning – a sudden spark, or energy
discharge, caused when electrical
charges jump between parts of a
cloud, between nearby clouds, or
between a cloud and the ground
KEY TERMS
 Tornado – a rapidly whirling, funnelshaped cloud that reaches down to
touch Earth’s surface
 Hurricane – a tropical storm that has
winds of about 119 kilometers per
hour or higher
 Storm surge – a “dome” of water that
sweeps across the coast where a
hurricane lands
STORMS
Storms involve sudden changes in air
pressure
In turn cause rapid air movements
Conditions that bring one kind of storm
often cause other kinds of storms in the
same area
Conditions that cause thunderstorms can also
cause tornados
HOW THUNDERSTORMS FORM
Thunderstorms form in large
cumulonimbus clouds
Also known as
thunderheads
HOW THUNDERSTORMS FORM
Most cumulonimbus clouds form on hot,
humid afternoons
Also form when warm air is forced upward
along a cold front
Many thunderstorms form in the spring and
summer in southern states or on the Western
Plains
LIGHTNING AND THUNDER
 During a thunderstorm areas of positive and
negative electric charges build up in the storm
clouds
 Lightning is a sudden spark, or electric
discharge, as these charges jump between parts
of a cloud
 Similar to the shocks you sometimes feel when
you touch a metal object on a dry day… but on a
MUCH larger scale!
LIGHTNING AND THUNDER
 A lightning bolt can heat the air near it to as much
as 30,000 degrees Celsius
 MUCH hotter than the sun’s surface
 The rapidly heated air expands suddenly and
explosively
 Thunder is the sound of the explosion
 Because light travels faster than sound, you see
lightning before you hear thunder
THUNDERSTORM DAMAGE
 Can cause severe damage
 The heavy rains associated with thunderstorms can flood
low-lying areas
 Lightning strikes can shatter tree trunks or start forest
fires
 When lightning strikes a person it is like a powerful
electric shock
 Can cause burns, unconsciousness, or even heart failure
FLOODS
 A major danger during thunderstorms is flooding
THUNDERSTORM SAFET Y
During thunderstorms, avoid places
where lightning may strike
Also avoid objects that can conduct
electricity, such as metal objects
and bodies of water
THUNDERSTORMS
Read Thunderstorms on pages 319-320 of
your textbook
TORNADOES
 One of the most frightening and destructive storms
 A rapidly whirling, funnel -shaped cloud that reaches down
from a storm cloud to touch Earth’s surface
 If it occurs over a lake or ocean it is known as a water spout
 Are usually brief
 May touch the ground for 15 minutes or less
 Only a few hundred meters across
 Wind speeds may approach 500 kilometers per hour
HOW TORNADOS FORM
Tornados most commonly
develop in think
cumulonimbus clouds
The same clouds that bring
thunderstorms
HOW TORNADOS FORM
Are most likely to occur
in spring and early summer
in late afternoon when the ground is warm
The Great Plains have the kind of
weather pattern that is likely to create
tornados
Warm, humid air mass moves north from the
Gulf of Mexico into the lower Great Plains
A cold, dry air mass moves south from Canada
TORNADO ALLEY
Tornados are rare in California
About 800 occur in the United States
every year
View figure 21 on page 321 of your
textbook
TORNADOES
Read Tornadoes on pages 320-321 of
your textbook
SNOWSTORMS
All year round, most
precipitation begins in clouds
as snow
If the air is colder than 0°C all
the way to the ground, the
precipitation falls as snow
SNOWSTORMS
In California, snowstorms generally occur only
at high elevations
Californians depend on snow to provide part of
their fresh water needs
 Snow that accumulates during the winter melts in the
spring and summer
 This snowmelt provides fresh water for a variety of
needs, including irrigation and electricity production
HURRICANES
A hurricane is a cyclone that has winds
of 119 kilometers per hour or higher
Form in the Atlantic, Pacific, and Indian
oceans
In the western Pacific Ocean, hurricanes
are called typhoons
HOW HURRICANES FORM
A hurricane begins over warm
ocean water as a low-pressure
area, or tropical disturbance
HOW HURRICANES FORM
A hurricane draws its energy from the warm,
humid air at the ocean’s surface
As the air rises from the clouds, more air is
drawn into the system
The lower the air pressure at the center of the
storm, the faster the winds blow toward the
center
HOW HURRICANES FORM
Hurricane winds are strongest in the
narrow band around the center of the
storm
At the center is a ring of clouds, called
the eyewall, that enclose the quiet “eye”
HOW HURRICANES MOVE
 Hurricanes last longer than other storms
 Usually a week or more
 During this time they can travel quite far
 Hurricanes that form in the Atlantic are steered by
the trade winds toward the Caribbean islands and
the southeastern United States
 After a hurricane passes over land, it no longer has
warm, moist air to draw energy from, and gradually
loses strength
HURRICANES
Read Hurricanes on pages 324-325 of
your textbook
CHECK FOR UNDERSTANDING
What weather conditions are most likely
to produce tornadoes?
Why do tornadoes occur most often in
“tornado alley?”
CHECK FOR UNDERSTANDING
What weather conditions are most likely to
produce tornadoes?
Think cumulonimbus clouds along a front
are the conditions most likely to produce
tornadoes.
Why do tornadoes occur most often in
“tornado alley?”
The area known as “tornado alley” often has
cold dry air meeting warm, moist air.
GUIDED PRACTICE
What is a hurricane?
How do hurricanes form?
GUIDED PRACTICE
What is a hurricane?
A hurricane is a tropical cyclone with
very high winds.
How do hurricanes form?
Hurricanes form when low pressure
systems form over warm water. The
rising warm, moist air adds energy to the
system, which eventually becomes a
hurricane.
INDEPENDENT PRACTICE
Complete Weather 8-4 Independent
Practice