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Activity 1
What Conditions Create Thunderstorms?
Think About It
Page F66
 Where are thunderstorms
most likely to occur in
the United States?
Why?
 How do thunderstorms
form?
Date
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Activity 1
What Conditions Create Thunderstorms?
Investigate Part A
Page F67
1a. Record all the
observations you made.
1b. Record the methods
you used to determine
the volumes of the two
balloons.
1c. What do the volumes
of the balloons tell you
about the density of air
in each balloon?
1d. How does air
temperature affect the
density of air?
2a. Record your
observations.
2b. Explain your
observations.
Date
Page
3
Activity 1
What Conditions Create Thunderstorms?
Investigate Part B
Pages F67-69
1a. On average, which
places experience the
most thunderstorms?
1b. On average, which
places experience the
fewest thunderstorms?
1c. How does the
frequency of thunderstorms
change from north to
Date
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4
south over the eastern
half of the nation?
1d. What is the relationship,
if any, between thunderstorm
frequency and mountainous
terrain?
1e. How does thunderstorm
frequency relate to proximity
to large bodies of water?
1f. How does the
thunderstorm frequency
on the East Coast compare
to the West Coast?
1g. List all the factors
that you think may
influence thunderstorm
frequency.
1h. Which of these factors
are present in or around
your community?
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2b. Draw a thick line to
Separate warm air masses
From cold air masses.
Describe how you
differentiated between
them.
2c. Describe what you
think happens when warm
and cold air masses meet.
Draw and label a diagram
to explain your reasoning.
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Activity 1
What Conditions Create Thunderstorms?
Digging Deeper
Pages F70-74
Warm air rises
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warm air is less dense than cold
air because molecules in warm air
are more active than the molecules
in cold air
Air masses can also be forced to
rise when they flow over
mountains or collide with other air
masses
There are four main mechanisms
that cause air to rise:
1. convective uplift
2. orographic uplift
3. frontal wedging
4. convergence
Convection
the transfer of heat by vertical
movements in the atmosphere as a
result of density differences
caused by heating from below
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Orographic
relating to mountains
Uplift caused when the wind
encounters a mountain range, the
mountain range acts as a barrier,
and forces the air upward
8
http://www.mhhe.com/biosci/genbio/tlw3/eBridge/Chp29/a
nimations/ch29/rain_shadow_formation.swf
Front
a narrow zone of transition
between air masses that contrast in
temperature and/or humidity
Frontal wedging
occurs when a cold air mass meets
a warm air mass, the more dense
cold air wedges or forces its way
beneath the less dense warm air
along a cold front
http://www.mesoscale.iastate.edu/agron206/animations/05_
cnWfronts.html
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http://www.classzone.com/books/earth_science/terc/content
/visualizations/es2002/es2002page01.cfm?chapter_no=visu
alization
Convergence
when winds blowing from
different directions meet head to
head, or converge, they have
nowhere to go but up
North America
air masses
cold air masses usually flow
southward and warm air masses
flow northward
Example
in winter a cold air mass is
warmed as it moves southeastward
from its snow-covered source
region to the bare ground of the
southern United States
Air masses
are classified by their temperature
and humidity:
•Continental: relatively dry air
masses that form over land
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•Maritime: relatively humid air
masses that form over the ocean
•Polar: cold air masses that form
at high latitudes (North)
•Tropical: warm air masses that
form at low latitudes (South)
Combinations
•Continental Polar (cold/dry)
•Continental Tropical (warm/dry)
•Maritime Polar (cold/humid)
•Maritime Tropical (warm/humid)
•Arctic Air (very cold/dry)
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Cold front
formed when a colder air mass
advances while the warmer air
retreats
Warm front
when the warmer air advances
while the colder air retreats
http://www.mesoscale.iastate.edu/agron206/animations/05_
cnWfronts.html
http://www.classzone.com/books/earth_science/terc/content
/visualizations/es2002/es2002page01.cfm?chapter_no=visu
alization
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Thunderstorm
caused by air rising along either
warm fronts or cold fronts (most
intense storms develop with cold
fronts)
The greater the temperature
contrast between two air masses
on either side of the cold front,
and the more rapidly the cold air
mass wedges under the warm-air
mass, the greater the chance that a
thunderstorm will form
It is a relatively small, short lived
weather system
Life cycle of
a thunderstorm
less than an hour, and has 3 stages:
1. cumulus
2. mature
3. dissipating
Cumulus stage
cumulus clouds build upward and
laterally
They developed where air ascends
as an updraft
As it reaches higher levels in the
atmosphere it expands and cools
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The cooling of the air causes some
of the water vapor in the air to
condense producing clouds
No precipitation occurs during the
cumulus stage of thunderstorm
development
Frontal wedging, orographic
uplifting, or converging surface
winds can strengthen an updraft
and cause cumulus clouds to build
vertically into thunderstorm
clouds
14
http://www.meted.ucar.edu/fire/s290/unit6/media/flash/43.3.1.pop1.htm
http://www.pals.iastate.edu/mteor/modules/subindex-c.html
Mature stage
the cumulus cloud becomes taller
and taller and eventually produces
rain, hail, or even snow
(precipitation is heaviest)
http://www.meted.ucar.edu/fire/s290/unit6/media/flash/43.3.2.pop1.htm
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http://www.pals.iastate.edu/mteor/modules/subindexm.html
Cumulonimbus
cloud
once precipitation begins, the
thunderstorm has entered its
mature stage
During the mature stage,
precipitation is heaviest, lightning
is most frequent, and hail and
tornadoes may develop
Dissipating stage
precipitation tapers off and ends,
clouds dissipate, and the chances
for severe weather decrease
dramatically
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http://www.pals.iastate.edu/mteor/modules/subindex-d.html
http://academic.cengage.com/resource_uploads/static_resou
rces/0534397719/1406/Ahrens15-1AirMassCloud.swf
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Activity 1
What Conditions Create Thunderstorms?
Check Your Understanding
Date
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Page F74
1. Describe a front.
2. Draw a diagram
Convection
Orographic uplift
3. Define thunderstorm.
4. Describe how an
approaching cold front
can promote the
development of
thunderstorms.
5. Describe the three
stages of a thunderstorm.
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Activity 1
What Conditions Create Thunderstorms?
Understanding and Applying
Page F75
1. In the central and
eastern parts of the U.S.,
what accounts for the
general increase in
thunderstorms as you go
south?
2. In the U.S., during
which season(s) do you
expect the most
thunderstorms to occur?
Explain.
3. Explain why there
are so many thunderstorms
in Louisiana and so few
thunderstorms in
Washington state.
4. Why is convective
uplift likely to be
strongest during the
warmest time of day?
5a. Sketch a cross section
of the sea-breezes in
Florida.
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5b. How does the meeting
of the sea breezes help
explain central Florida’s
high thunderstorm
frequency?
6. Why might you see
scattered cumulus clouds
on a warm summer
afternoon?
7. Is severe weather
inevitable in your
community? Explain.
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What Conditions Create Thunderstorms?
Inquiring Further
Page F76
1. Personal memories
of thunderstorms
Ask members of your
family or neighbors about
the most memorable
thunderstorms they ever
witnessed in your
community:
•What do they
remember about the
appearance,
development and how
long the storm lasted?
•What thunderstorm
hazards or impacts
have they experienced?
•What was the worst
thunderstorm they ever
experienced?
3. Severe weather events
in your community
Research information on
severe weather events that
have affected your
community in the past.
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