Transcript Severe Storms

Severe Storms
Ch 20.3
Thunder Storms
• A thunderstorm is a storm that generates
lightning and thunder
How Often Do
Thunderstorms Occur?
• At any given time, there are an estimated
2000 thunderstorms in progress right now on
Earth
• About 45,000 thunderstorms take place each
day
• The United States experiences about
100,000 thunderstorms each year
• Where do these most frequently occur in the
US?
– Florida and the Gulf Coast region
Development Of
Thunderstorms
• Thunderstorms form when warm, humid air
rises in an unstable environment
• Three Stages
– Cumulus Stage
– Mature Stage
– Dissipating Stage
Cumulus Stage
• 1st stage
• Strong updrafts,
supply moist air
• What happens
with each new up
draft?
– The cloud
becomes taller
Fig 17A
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Mature Stage
• 2nd stage
• Precipitation becomes too large for updrafts
and heavy precipitation is released from the
cloud
• The most active stage of a thunderstorm.
• Gusty winds, lightning, heavy precipitation,
and sometimes hail are produced during this
stage
Mature Stage
Fig 17B
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Dissipating Stage
• Final stage
• Downdrafts dominate updrafts and
causes the storm to die
Tornadoes
• Tornadoes are violent windstorms that
take the form of a rotation column of
air.
How Often Do
Tornadoes Occur?
• Most tornadoes form in association with severe
thunderstorms
• In the United States, about 770 tornadoes are
reported each year
• The frequency of tornadoes is greatest from April
through June
Tornado Intensity
• Low pressure within a tornado
causes air near the ground to rush
into a tornado from all directions
• As the air streams inward, it spirals
upward around the core
• Eventually, the air merges with the
airflow of the cumulonimbus cloud
that formed the storm
Fujita Tornado
Intensity Scale
Interactive Tornado
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Tornado Safety
• Tornado watches alert people to the possibility of
tornadoes in a specified area for a particular time
• Tornado warning is issued when an actual tornado
has been sighted in your area
Hurricanes
• Tropical cyclones that produce winds of at
least 119 kph (75 mph) are known in the
United States as hurricanes
• Most powerful storms on Earth
• When hurricanes hit land, why do they lose
so much energy?
– Friction
– No warm, moist air
Hurricanes
• In September 1999, Floyd brought flooding rains,
high winds, and rough seas to a large portion of the
Atlantic coast
• More than 2.5 million people evacuated their homes
• Torrential rains caused devastating inland flooding
• Floyd was the deadliest hurricane to strike the U.S.
mainland since Hurricane Agnes in 1972
• Most of the deaths caused by Hurricane Floyd were
the result of drowning from floods
Floyd
Fig 20
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Hurricane Floyd 1999
Hurricane Floyd 1999
Hurricane Floyd 1999
Rocky Mount NC 7am
Same Place 3pm
Occurrence Of Hurricanes
• Most hurricanes form between about 5 and 20
degrees north and south latitude. The North Pacific
has the greatest number of storms, averaging 20
per year
Development Of Hurricanes
• Hurricanes develop most often in the late summer
when water temperatures are warm enough to
provide the necessary heat and moisture to the air
• The eye is a zone of scattered clouds and calm
winds at the center of a hurricane
Hurricane Cross Section
Fig 21
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Hurricane Intensity
• The intensity of a hurricane is described using the
Saffir-Simpson scale
• A storm surge is the abnormal rise of the sea along
a shore as a result of strong winds
Saffir-Simpson Scale
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