Thunderstorms, Tornadoes,
and Hurricanes
Mr. Q and Mrs.Wolfe
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Where are Thunderstorms
common?
Average number of days (per year) that a thunderstorm occurs (Fig 10.18)
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Thunderstorms
• Common in the summer season
• Usually not part of a larger-scale weather
system
• Short-lived, rarely severe
• Occur when a warm front and a cold front
meet.
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Cumulus Stage
Updrafts dominate
Precipitation begins to fall
Mature Stage
Lightning, Thunder, Peak strength
Clouds may reach 45-50,000 ft
Dissipating Stage
Downdrafts dominate
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Stages of an Air Mass
Thunderstorm
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Severe Thunderstorms
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What is a Thunderstorm?
• A thunderstorm is a storm with
thunder, lightning, and often
heavy rain and strong winds.
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Structure of Severe Thunderstorms
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Features of Severe
Thunderstorms
• Cumulus clouds build up to
form cumulonimbus clouds.
• Heavy rain or hail forms.
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Features of Severe
Thunderstorms
• Downburst
– A highly localized, very strong downdraft
– Hits ground and spreads out in a circular direction
• Microburst
– A downburst with winds only extending less than 4
km in any direction
– Winds can be as high as 150 kt.
• Extremely hazardous to Aircraft
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a) Strong headwind/lift
b) Enhanced lift ends, plane pushed downward
c) Strong tailwind stalls plane, crash ensues
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Delta 191 Crash At DFW, 1985
• Microburst produced increased lift initially
• Plane then experienced dramatic tailwind and
decrease in lift
• Crash killed 136 / 167
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Supercell Thunderstorm
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Thunderstorms
• Line of storms that
form just ahead of a
cold front or some
distance ahead
(100-300 km)
• Can be over 600
miles long
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Lightning and Thunder
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• Lightning - An electrical discharge formed
by charge seperation within thunderstorms
or between thunderstorms and the ground
• Thunder – Shock wave produced by
lightning
– Lightning heats air to around 30,000 K
– Air expands very rapidly, creates loud noise
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Lightning Characteristics
• Globally, about 100 flashes occur each second
• Most lightning (~80%) occurs between clouds.
Only about 20% is between cloud and ground
• About 100 deaths/year in U.S. by lightning
strikes (second most by a natural phenomena)
– Most in Florida
– Hazardous activities: golf, sailing, walking in
open area
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Thunder
• Travels about 1100 ft./sec
– If lightning flash is 3 miles away, it will
take about 9 seconds for thunder to be
heard
– If lightning hits you, you will
instantaneously hear thunder before
vaporizing
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– Net negative charge builds up near bottom of
cloud
– This attracts net positive charge buildup from
ground
– When charge seperation is too great, lightning
strike will occur
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Types of Lightning
• Cloud to cloud
• Cloud to ground
• Ball lightning
– Appears as a luminous sphere
– Cause unknown
• Sheet lightning
– Lightning inside of clouds, appears white
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Ribbon Lightning
Bead Lightning
Sheet Lightning
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St. Elmo’s Fire?
wvlightning.com/types.shtml
C.C. Hennon, UNC Asheville
Ball lightning
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Lightning Safety
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•
•
•
•
•
Don’t go golfing
Don’t talk on the telephone (cell OK)
Don’t go for a walk
Don’t find an isolated tree to stand under
If your hair stands on end, lightning is
about to strike
– Crouch down (don’t lay down) as low as
possible
• Stay away from the bathroom and
windows
• Cars are OK
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Tornadoes
• Rapidly rotating,
small-scale winds that
form within severe
thunderstorms
• Many different
shapes; last
anywhere from a few
seconds to several
hours
spinning ice skater
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Tornado Occurence
• Have occurred in all 50 U.S. states, but
rare in Western U.S., Alaska, Hawaii
• Form during every month of the year, but
75% form during months of March – July
• Globally, U.S. has highest occurrence of
tornadoes – WHY?
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Top Number: Number of tornadoes over a 25-year period (1953-2004)
Bottom Number: Average number per year per 10000 sq. km
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Tornadic Winds and Classification
• Tornadoes are classified on the Fujita
Scale
F-0 : 65-85 mph
F-1 : 86-110 mph
F-2 : 111-135
F-3 : 136-165
F-4 : 166-200
F-5 : 200+
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F1 Tornado
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F2 Tornado
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F3 Tornado
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F4 Tornado
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F5 Tornado
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Tornado Formation
• Updrafts in thunderstorms must rotate
– Called a mesocyclone
• As mesocyclone stretches vertically, its
diameter shrinks and it spins faster
• Tornado forms within the mesocyclone
and descends towards the surface (not
understood why)
• If condensation occurs within funnel, a
funnel cloud will be seen
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Observing Tornadoes
First sign (visually) of a tornado is the appearance of a
wall cloud
Rotating cloud underneath the main cell
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stormchasing.ca/_wsn/page11.html
C.C. Hennon, UNC Asheville
Portable Radar Units
(aka crazy people)
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Warning For Tornadoes
• Tornado Watch – Conditions are favorable
for tornado formation. Monitor local
weather conditions and broadcasts
• Tornado Warning – Tornado and/or funnel
cloud and/or hook echo has been spotted
in the area
– Take cover immediately
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Anatomy of a Tornadic
Thunderstorm
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Hurricanes
• A hurricane is a tropical storm with strong
winds.
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Eye
• The center of the storm is called the eye.
• It is calm and serene.
• No winds.
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Parts of a Hurricane
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How a Hurricane Forms
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