Transcript Severe Weather Section 13.2

Section 13.2
Severe Weather
Objectives
Explain why some thunderstorms are more
severe than others.
Recognize the dangers of severe weather,
including lightning, hail, and high winds.
Describe how tornadoes form.
Section 13.2
Severe Weather
All thunderstorms produce wind, rain, and
lightning, which can have dangerous and
damaging effects under certain circumstances.
Review Vocabulary
air mass: large body of air that takes on
the characteristics of the area over
which it forms
Section 13.2
Severe Weather
New Vocabulary
supercell
tornado
downburst
Fujita tornado
intensity scale
Section 13.2
Severe Weather
Weather Cells
The increasing instability of the air
intensifies the strength of a storm’s
updrafts and downdrafts, which makes
the storm severe.
Section 13.2
Severe Weather
Weather Cells
Supercells- rotating updrafts.
Severe thunderstorms can develop into
self-sustaining, extremely powerful storms
called supercells.
These furious storms can last for several
hours and can have updrafts as strong as
240 km/h.
Section 13.2
Severe Weather
Weather Cells
Supercells
An anvil-shaped cumulonimbus
cloud is characteristic of many
severe thunderstorms.
Section 13.2
Severe Weather
Strong Winds
Violent downdrafts that are concentrated in
a local area are called downbursts.
Based on the size of the area they affect,
downbursts are classified as either
macrobursts or microbursts.
Section 13.2
Severe Weather
Hail
Hail is precipitation in the form of balls or
lumps of ice. It forms because of two
characteristics common to thunderstorms.
Section 13.2
Severe Weather
Hail
For hail to form,
water droplets rise to the heights where the
temperature is below freezing, encounter ice
pellets, and freeze on contact with the pellets,
which causes the ice pellets to grow larger.
The second characteristic that allows hail to form
is an abundance of strong updrafts and
downdrafts moving side by side within a cloud
which tosses the hail up and down repeatedly.
Section 13.2
Severe Weather
Tornadoes
A tornado is a violent, whirling column of
air in contact with the ground.
Before a tornado reaches the ground, it is
called a funnel cloud.
Section 13.2
Severe Weather
Tornadoes
Development of tornadoes
A tornado forms when wind speed and
direction change suddenly with height, a
phenomenon associated with wind shear.
Although tornadoes rarely exceed 200 m in
diameter and usually last only a few
minutes, they can be extremely destructive.
Section 13.2
Severe Weather
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Section 13.2
Severe Weather
Tornadoes
Tornado classification
The Fujita tornado intensity scale, which
ranks tornadoes according to
1.their path of destruction,
2.wind speed,
3. and duration,.
The scale ranges from F0 –F5
Section 13.2
Severe Weather
Tornadoes
Tornado distribution
Most tornadoes—especially violent ones—form
in the spring during the late afternoon and
evening, when the temperature contrasts
between polar air and tropical air are the
greatest. This type of large temperature
contrast occurs most frequently in the central
United States.
Section 13.2
Severe Weather
Tornadoes
Tornado distribution
Many of the more than 700 tornadoes that
touch down in the United States each year
occur in a region called “Tornado Alley,”
which extends from northern Texas through
Oklahoma, Kansas, and Missouri.
Section 13.2
Severe Weather
Tornadoes
Tornado safety
If you are caught in a
tornado, take shelter
in the southwest
corner of a basement,
a small downstairs
room or closet, or a
tornado shelter.
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Study Guide
Key Concepts
Section 13.2 Severe
Weather
All thunderstorms produce wind, rain,
and lightning, which can have dangerous and
damaging effects under certain circumstances.
 Intense rotating updrafts are associated
with supercells.
 Downbursts are strong winds that result in
damage associated with thunderstorms.
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Study Guide
Key Concepts
Section 13.2 Severe
Weather
 Hail is precipitation in the form of balls or
lumps of ice that accompany severe storms.
 The worst storm damage comes from a
vortex of high winds that moves along the
ground as a tornado.
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The Nature of Storms
13.2 Section Questions
The strongest thunderstorms develop under
highly stable atmospheric conditions.
a. true
b. false
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The Nature of Storms
13.2 Section Questions
Which type of precipitation requires strong
updrafts and downdrafts to exist side by side
in a cloud?
a. rain
b. snow
c. hail
d. sleet
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The Nature of Storms
13.2 Section Questions
How do tornadoes form?
Answer: The rotation of a tornado begins
as a result of wind shear, wind at different
levels of the atmosphere blowing in different
directions or at different speeds. The horizontal
rotation is then tilted to a vertical position by
thunderstorm updrafts. A tornado forms if the
rotating column extends to the ground.
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The Nature of Storms
Standardized Test
Practice
Why do hailstones consist of concentric layers?
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The Nature of Storms
Standardized Test
Practice
Possible answer: The layers form as
hailstones are tossed in a thunderstorm by
strong updrafts. A new layer of ice is added
each time the hailstone moves into a level of
supercooled water droplets. These water
droplets exist at temperatures that are well
below the normal freezing point of water and
so change to ice as soon as they come into
contact with the hailstone.