Download Tornadoes Module 1 - Northern Illinois University

Tornadoes Module 1
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- Tornado Definition -
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Tornadoes Module 1
- Tornado Facts y Cool air overrides a layer of warm air, forcing the
warm air to rise rapidly
y Spawned by thunderstorms or hurricanes
y Does not have to be raining
y May change direction or double back suddenly
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- Tornado Facts (continued)
y Most destructive of all storms
y Can occur anywhere in the world
y Most frequent in the United States
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- Danger Signs y The wind may become very still
 Clear, sunlit skies may be behind a tornado
y Sky may appear green
y Funnel may be invisible until it accumulate
enough debris
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Tornadoes Module 1
- Danger Signs (continued)
y Lower parts of tornadoes also can assume
the color of dust and debris
 A tornado passing across western or central
Oklahoma may look red
y Usually appear grey
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- Waterspouts y Tornadoes that travel across lakes or oceans
y Low pressure draws water away from the
water’s surface
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Tornadoes Module 1
- Hazards y Winds may exceed 300 mph
y Roof gables and eaves act as airplane wings
y Combination creates upward force capable
of removing roofs
y Building damage allows wind entry
(exceeding 40 pounds per sq foot)
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- Hazards (continued)
y Damage also caused by air pressure reductions
y Air pressure at a tornado’s center is
approximately 20 percent less than at sea level
y Structures collapse outward when subject to
pressure drops of this magnitude
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- Hazards (continued)
y Typical destructive path
 1/3 mile (approx. 570 yards)
 15 miles long
y Exceptions do occur
 Tornado in 1917 traveled 350 miles lasting well over 7 hours
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Tornadoes Module 1
- Fujita Scale y Measures tornado intensity
 Analyzes damage
 Assigns estimated wind speed
y Six levels
 F0 least destructive
 F5 most destructive
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- Tornado Paths -
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Tornadoes Module 1
- Enhanced Fujita (EF) Scale y Refines existing Fujita Scale
 Ranges in wind speed will be more accurate
 28 damage indicators will estimate wind speed
y Rates tornadoes from 0 to 5
y Fully implemented February 2007
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- Scale Differences -
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- Fujita Scale Limitations y Subjective based solely on damage
y No recognition of construction quality differences
y Difficult to apply without damage indicators
y Subject to bias
y Based on the worst damage
y Overestimates wind speeds of F3s and worse
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Tornadoes Module 1
- Fujita Scale Misuses y Reliance on estimated wind speeds
y Limited damage descriptions
y No recognition of building subtleties
y Judged by cloud appearance qualities
y No allowance for weak structures
 Mobile homes
 Modified homes
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- Tornado Risk y Mostly March through August
y Often occur afternoons and evenings
y Over 80 percent between noon and midnight
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Tornadoes Module 1
- Annual Occurrence -
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- Recent Tornadoes -
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- Tornado Risk y Short warning times
y Keys to surviving a tornado:
 Advance planning
 Quick response
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- Additional Online Resources y National Geographic, “Chasing Tornadoes”

http://magma.nationalgeographic.com/ngm/0404/sights_n_s
ounds/media2.html
y MSNBC Interactive, “Birth of a Tornado”

http://www.msnbc.com/modules/tornado/default.asp
y NOAA Storm Prediction Center, “The Enhanced
Fujita (EF) Scale”

http://www.spc.noaa.gov/efscale/
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