Chapter 13 – The Nature of Storms

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Transcript Chapter 13 – The Nature of Storms

Chapter 13
The Nature of Storms
SES5. Students will investigate the interaction of insolation and Earth
systems to produce weather and climate.
e. Describe the hazards associated with extreme weather events and
climate change (e.g., hurricanes, tornadoes, El Niño/La Niña, global
Chapter 13 – The Nature of Storms
• 13.2 Severe Weather
• 13.3 Tropical Storms
13.2 Severe Weather
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.
13.2 Severe Weather
• Main Idea
– All thunderstorms produce wind, rain, and
lightning, which can have dangerous and
damaging effects under certain
• Review Vocabulary
– air mass: large body of air that takes on the
characteristics of the area over which it forms
Weather Cells
• All thunderstorms are not created
• The increasing instability of the air
intensifies the strength of a storm’s
updrafts and downdrafts, which makes
the storm severe.
Weather Cells
• Supercells
– Severe
thunderstorms can
develop into selfsustaining, extremely
powerful storms
called supercells.
– These furious storms
can last for several
hours and can have
updrafts as strong as
240 km/h (150 mph).
Weather Cells
• Supercells
– An anvil-shaped cumulonimbus cloud is
characteristic of many severe thunderstorms.
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.
– Macroburts: path of destruction up to 5 km
wide, wind speeds >200 km/h, last up to 30
– Microbursts: path of destruction <3 km, winds
>250 km/h, last <10 mins.
• Hail is precipitation in the form of balls or lumps
of ice. It forms because of two characteristics
common to thunderstorms.
• For hail to form, water droplets rise to the
heights of a cumulonimbus cloud 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.
• 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.
• Development of
– A tornado forms
when wind speed
and direction
change suddenly
with height, a
associated with
wind shear.
– Although tornadoes
rarely exceed 200 m in
diameter and usually
last only a few minutes,
they can be extremely
Tornado classification
The Fujita tornado intensity scale, which ranks
tornadoes according to their path of destruction,
wind speed, and duration, is used to classify
Fujita Tornado Intensity Scale
• 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.
– 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
• 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.
13.2 Section Summary
• Intense rotating updrafts are associated
with supercells.
• Downbursts are strong winds that result in
damage associated with thunderstorms.
• Hail is precipitation in the form of balls or
lumps of ice that accompany severe
• The worst storm damage comes from a
vortex of high winds that moves along the
ground as a tornado.
13.2 Section Questions
The strongest thunderstorms develop under
highly stable atmospheric conditions.
a. true
b. false
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
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.
13.3 – Tropical Storms
• Real-World Link: If you try
mixing cake batter in a shallow
bowl, you might find that a low
speed works well, but a high
speed creates a big mess.
Tropical cyclones form from
processes similar to other storm
systems, but their high winds can
bring devastation to locations in
their path.
13.3 – Tropical Storms
• Main Idea
– Normally peaceful, tropical oceans are
capable of producing one of Earth’s most
violent weather systems—the tropical
• Review Vocabulary
– Coriolis effect: caused by Earth’s rotation,
moving particles, such as air, are deflected to
the right north of the equator, and to the left
south of the equator
13.3 – Tropical Storms
• Objectives
– Identify the conditions required for tropical
cyclones to form.
– Describe the life cycle of a tropical cyclone.
– Recognize the dangers of hurricanes.
Overview of Tropical Cyclones
During summer and fall, the tropics
experience conditions ideal for the
formation of large, rotating, lowpressure tropical storms called tropical
Overview of Tropical Cyclones
Cyclone location
Favorable conditions for cyclone
formation exist in all tropical oceans
except the South Atlantic Ocean and
the Pacific Ocean off the west coast of
South America.
Overview of Tropical Cyclones
Cyclone formation
Tropical cyclones require two basic
conditions to form: an abundant supply
of warm ocean water and some sort of
mechanism to lift warm air and keep it
Overview of Tropical Cyclones
Cyclone formation
The first indication of a building tropical
cyclone is a moving tropical disturbance.
When a disturbance over a tropical ocean
acquires a cyclonic circulation around a
center of low pressure, it has reached the
developmental stage and is known as a
tropical depression.
Overview of Tropical Cyclones
Cyclone formation
When wind speeds around the low-pressure center of a
tropical depression exceed 65 km/h, the system is
called a tropical storm.
If air pressure continues to fall and winds around the
center reach at least 120 km/h, the storm is officially
classified as a cyclone.
Overview of Tropical Cyclones
Cyclone formation
Once winds reach at least 120 km/h,
another phenomenon occurs—the
development of a calm center of the
storm called the eye.
The eye of the cyclone is a span of
30 to 60 km of calm weather and blue
Overview of Tropical Cyclones
Cyclone formation
The strongest winds in a hurricane are
usually concentrated in the eyewall—a
tall band of strong winds and dense
clouds that surrounds the eye.
Overview of Tropical Cyclones
Cyclone formation
A hurricane will last until it can no
longer produce enough energy to
sustain itself. This usually happens
when the storm has moved either over
land or over colder water.
Overview of Tropical Cyclones
Tropical cyclone movement
Like all large-scale storms, tropical
cyclones move according to the wind
currents that steer them.
Hurricane Hazards
The Saffir-Simpson
hurricane scale
classifies hurricanes
according to wind
speed, potential for
flooding due to the
effect on the height of
sea level, and potential
for property damage.
Hurricane Hazards
Hurricanes can cause extensive
damage, particularly along coastal
areas, which tend to be where human
populations are the most dense.
Hurricane Hazards
Much of the damage caused by
hurricanes is associated with violent
Hurricane Hazards
Storm surge
A storm surge occurs when hurricane-force
winds drive a mound of ocean water toward
coastal areas where it washes over the
Hurricane Hazards
Hurricane advisories and safety
The National Hurricane Center issues a
hurricane warning at least 24 hours
before a hurricane is predicted to strike.
Awareness, combined with proper
safety precautions, has greatly reduced
death tolls associated with hurricanes in
recent years.
13.3 – Summary
• Cyclones rotate counterclockwise in the northern
• Cyclones are also known as hurricanes & typhoons.
• Cyclones go through the same stages of formation &
dissipation as other storms.
• Cyclones are moved by various wind systems after they
• The most dangerous part of a tropical cyclone is the
storm surge.
• Hurricane alerts are given at least 24 hours before the
hurricane arrives.
13.3 – Review Questions
At what latitudes do tropical cyclones
usually form?
a. between 0 and 5
b. between 5 and 30
c. between 30 and 50
d. between 50 and 70
13.3 – Review Questions
What weather condition usually exists within the
eye of a hurricane?
a. high wind
b. calm wind
c. tornadoes
d. lightning
13.3 – Review Questions
What is the source of a hurricane’s energy?
Answer: A hurricane’s energy comes from
the warm water over which it develops. As
ocean water evaporates, some heat is taken
from the ocean. The water vapor then rises
high into the atmosphere. The heat that was
taken from the ocean is released to the
atmosphere as the water vapor condenses.