17
Activity 1
What Conditions Create Thunderstorms?
Think About It Date
Page F66 Page
· Where are thunderstorms
most likely to occur in
the United States?
Why?
· How do thunderstorms
form?
Activity 1
What Conditions Create Thunderstorms?
Investigate Part A Date
Page F67 Page
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.
Activity 1
What Conditions Create Thunderstorms?
Investigate Part B Date
Pages F67-69 Page
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
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?
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.
Activity 1
What Conditions Create Thunderstorms?
Digging Deeper Date
Pages F70-74 Page
Warm air rises 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
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
http://www.mhhe.com/biosci/genbio/tlw3/eBridge/Chp29/animations/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
http://www.classzone.com/books/earth_science/terc/content/visualizations/es2002/es2002page01.cfm?chapter_no=visualization
Convergence when winds blowing from different directions meet head to head, or converge, they have nowhere to go but up
North America cold air masses usually flow
air masses 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
•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)
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=visualization
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 less than an hour, and has 3 stages:
a thunderstorm 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
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
http://www.meted.ucar.edu/fire/s290/unit6/media/flash/4-3.3.1.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/4-3.3.2.pop1.htm
http://www.pals.iastate.edu/mteor/modules/subindex-m.html
Cumulonimbus once precipitation begins, the
cloud 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
http://www.pals.iastate.edu/mteor/modules/subindex-d.html
http://academic.cengage.com/resource_uploads/static_resources/0534397719/1406/Ahrens15-1AirMassCloud.swf
Activity 1
What Conditions Create Thunderstorms?
Check Your Understanding Date
Page F74 Page
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.
Activity 1
What Conditions Create Thunderstorms?
Understanding and Applying Date
Page F75 Page
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.
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.
What Conditions Create Thunderstorms?
Inquiring Further Date
Page F76 Page
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.