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Bell Ringer
•Cumulonimbus clouds
have a tall structure and
a flat base. What are
they usually associated
with?
Severe Storms
Thunderstorms
and Tornadoes
What do you know
about
Thunderstorms?
Have you ever seen
a small whirlwind
carry dust or leaves
upward on a hot
day?
Thunderstorms
• A storm that generates lightning and
thunder.
• Frequently produce gusty winds,
heavy rain, and hail.
• Associated with cumulonimbus clouds
• Lightning: results
from the build up and
discharge of electric
energy between
positively (ground)
and negatively
(clouds) charged area
• Thunder: the sound
of rapidly expanding
gases usually
associated with
lightning
How common
are
thunderstorms?
Occurrence of Thunderstorms
• Estimated 2000 in progress
on Earth at any time
• About 45,000 take place each
day
• More than 16 million occur
annually around the world
• The greatest number occur in
the tropics
• Atmospheric Conditions
needed:
– Warm, moist air, uplift, and
instability
Development of Thunderstorms
• Form when warm, humid air rises in an
unstable environment
• 3 Stages
– Cumulus Stage
– Mature Stage
– Dissipating Stage
Cumulus Stage
• Warm, moist air is supplied to the cloud by
strong updrafts
• Cloud grows vertically
Mature Stage
• Begins within 1 hour of the initial updraft
• Most active stage
• Heavy precipitation, gusty winds, lightening,
and sometimes hail are produced
Dissipating Stage
• Cloud begins to evaporate
• The cooling effect of the falling precipitation
and the flowing in of colder air from high
above causes the storm to die down
Thunderstorm Development
The Thunderstorm Recipe Video
What do you know
about tornadoes?
Tornadoes
• Violent low pressure windstorms that take the
form of a vortex.
• Vortex- spinning/rotating, often turbulent,
column of air
• The vortex extends downward from a
cumulonimbus cloud – produce rain and hail
• Move counterclockwise
Occurrence of Tornadoes
• About 770 tornadoes are
reported in the United States
each year
• Frequency is greatest between
April and June
• Associated with Severe
Thunderstorms
Which states in the U. S. do
you think get the most
tornado touchdowns?
Development of Tornadoes
• Most form in association with
severe thunderstorms
• The formation of the
mesocyclone is an important
process in tornado formation.
National Geographic Tornadoes Video
Mesocyclone
• A vertical cylinder of rotating air
that develops in the updraft of a
thunderstorm
Formation of a Mesocyclone
How Tornadoes Form Video
Tornado Intensity
• Pressure within tornadoes can be as much
as 10% lower than the pressure outside
the storm.
• Intensity is measured on the Fujita Scale
Fujita Scale
•
•
•
•
•
First introduced in 1971 – Tetsuya Fujita
Measures the strength of tornadoes
Based on storm damage
Ranges from F0-F5
Updated 2007 – Enhanced Fujita
– Updated wind speeds match damaged caused by
tornado
Fujita Tornado Intensity Scale
Enhanced Fujita Scale
Fujita Tornado Scale
Category
Winds (MPH)
Winds (KPH)
Damage
F0
<73
<116
Light damage
F1
73-112
116-180
Moderate Damage
F2
113-157
181-254
Considerable Damage
F3
158-206
254-332
Severe Damage
F4
207-260
333-419
Devastating Damage
F5
>260
>419
Incredible and Speechless
Damage
Watch vs.
Warning
Tornado Watch
• Alert people to the possibility
of tornadoes in a specified
area for a particular time
• Conditions are right for a
tornado to happen
Tornado Warning
• Issued when a tornado has
actually been sighted in an
area or;
• Indicated by weather radar
Let’s Review!
Exit Ticket
Severe Storms
Causes
Effects
Thunderstorms
a)
b)
Tornadoes
c)
d)