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Meteorology
Unit 5 - Tornadoes
What is a tornado?
• Tornado – a violently destructive windstorm,
characterized by a long, funnel-shaped cloud
extending toward the ground and made visible
by condensation and debris
• Almost all tornadoes rotate counterclockwise
around a central core of low pressure
• A few have been seen rotating clockwise, but
this is extremely rare
Tornado AKA
• Also called twisters or cyclones
• Funnel cloud – tornado that has not reached
the ground
• Waterspout – a rotating column of air over a
body of water
Tornado Structure
• Diameter between 300 – 2,000 feet
• Few have been over 1 mile wide
– 2004, Nebraska – 2.5 miles wide
• Usually travel 20-50 mph, some clocked at 70+
mph
• Usually only last a few minutes and have an
average path length of 4 miles
– 1917, tornado traveled 292 miles through Illinois
and Indiana over a 7 hours
Tornado Life Cycle
• Dust-whirl Stage – dust swirling up from the ground
marks the tornadoes circulation and a short funnel
extends from thunderstorm’s base
• Organizing Stage – tornado increases in intensity,
funnel extends downward
• Mature Stage – Funnel reaches its greatest width and
contacts ground. Vertical. This is the stage where the
most damage occurs
• Shrinking Stage – decrease in funnel width, narrowing
in path, tornado may tilt. Still can cause severe
damage.
• Decay Stage – tornado looks rope-like and eventually
dissipates
Tornado Formation
• Form with intense thunderstorms and
conditionally unstable environments
• Usually with supercell storms with strong wind
shear
• Rotating air may begin in the storm and work
downward or begin on the ground and work
upwards
• Fueled by convective circulation of warm updrafts
and cold downdrafts
Vortex Tube
• Strong updrafts form a vortex tube. If updrafts
tilt and pull the vortex tube into the storm, it
becomes a rotating air column within the
storm
Vortex Tube
Tornado Occurrence
• Occur in many parts of the world, but most
common in the United States
– USA averages more than 1,000 yearly
– Record 1722 tornadoes in 2004
• Greatest number occur in Tornado Alley in
central US
• Tornado Alley often provides the right
atmospheric conditions
Tornado Occurrence
• Most tornadoes occur between March and
July
• Greatest number occur in May, but the
strongest tornadoes usually occur in April
• Most frequent between 4:00-6:00pm
Tornado Measurement
• Fujita Scale – original scale, wind speed was
estimated based on damage to surface
• Enhanced Fujita Scale (EF Scale) – using 28
different criteria, still uses damage to determine
wind speed
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EF0: 65-85mph
EF1: 86-110mph
EF2: 111-135mph
EF3:136 – 165mph
EF4: 166-200mph
EF5: >200mph
• The majority of tornadoes are weak at EF0 or
EF1. The United usually has one or two EF5
tornadoes each year
Strange Occurrences
• People, animals, and machinery picked up,
carried, and (mostly) safely deposited miles away
– Railroad coach with 117 passengers was picked up and
moved 80 feet
– Schoolhouse was demolished and the 85 students
inside were carried 300+ feet with no fatalities
– Showers of fish or frogs have fallen over land after
tornadoes took them from water
• Chickens losing all their feathers
• A piece of straw driven through a metal pipe
• Frozen hot dogs being driven though concrete
walls
What do I do?
• Seek shelter, preferably in a basement or interior
room
• Stay away from windows
• Find a shield, like a mattress
• If outside, lay flat in a depression or ravine and
cover your head
• If in a vehicle, do not try to outrun the tornado.
Stop and try to move perpendicular to the
tornado.
• Listen to the radio/TV for tornado watches and
warnings
Tornado Myths
• Opening windows will keep the house from
exploding
– Was once thought to help equalize pressure
– Now known it actually increases pressure on the
opposite wall and increases chance the building will
collapse
– Damage is mostly caused by wind and debris, not
pressure
• Take shelter under a highway overpass or bridge
– Winds are actually funneled and strengthened by the
overpass/bridge structure