Download Severe Storms

Severe Storms
Most Violent weather is associated with
low pressure systems because air of
different properties mixes there
• Thunderstorms
• Tornadoes
• Hurricanes
• Winter Storms
Thunderstorms
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Flash Flooding
Hail
Lightning
Downbursts
Tornadoes
Flash Flooding
Lightning
Thunder
Thunder and Lightning
• Superheated air expands and creates shock
wave
• Can be heard up to 30 miles away
• Flash-Sound Interval: 5 sec/mi (3 sec/km)
• Not one second = one mile
• “Heat Lightning” is ordinary lightning
illuminating the clouds
Lightning Rods
• Benjamin Franklin, 1752
• Do not allow lightning strikes to be
conducted to the ground
• Pointed shape allows excess charge to
bleed harmlessly into the atmosphere
(corona discharge)
Mammatus
Squall Line
The Fujita Scale
Based on Damage and Engineering Studies
• F0
40-73 mph
29%
• F1
74-112 mph
40%
• F2
113-157 mph 24%
• F3
158-206 mph 6 %
• F4
207-260 mph 2 %
• F5
261-318 mph <1 %
• F6?
How to identify?
Conditions for Tornado
Formation
• Energy Source (convection or uplift)
– Cold Front and Squall Line
– Supercells and Mesocyclones
• Vorticity (something to create a spin)
• Usually but not always spin according to
Coriolis Effect
• Spin is indirectly connected - inherited from
larger weather systems
Mesocyclone
Mesocyclone, Door County, August 2007
Door County Tornado, August 1998
Door County Tornado, August 1998
Door County Tornado, August 1998
Door County Tornado, August 1998
Door County Tornado, August 1998
Door County Tornado, August 1998
Door County Tornado, August 1998
Langlade County Tornado 2007
Where Tornados Occur
• U.S. and Canada probably have most
severe storms
• Cool Canadian air meets warm, moist Gulf
air
• Highest reported frequency by area is
Britain
• Other places: India, Australia, China
Tornado Myths
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Take shelter in the southwest corner
Take shelter under a bridge or overpass
Open windows to equalize pressure
Buildings explode from pressure drop
Tornados avoid rivers, hills, mountains
Certain localities are “protected”
Tornados avoid cities
Should you attempt to evade?
Fort Worth, Texas, March 28, 2000
Tornadoes do not avoid cities
Things Often Mistaken For
Tornadoes
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Heavy Precipitation
Downbursts
Dust Devils
Cold Funnels
• If There’s No Evidence of Rotation, It’s Not
a Tornado
Virga
Downburst, May 1994
Downburst Damage, Ontario
Dust Devil
Cold Funnels
Cold Funnels
Hurricanes
• Hurricane: Atlantic and East Pacific
• Typhoon: West Pacific
• Cyclone: Indian Ocean
• Intense Low-Pressure Systems
• Need 60 m (200 feet) of ocean water at
26.5 C or warmer to form
World Hurricane Tracks 19952003
Hurricane Forming Regions
Hurricane-Free Regions
• No Coriolis effect at equator, hence no
hurricanes within 5 degrees of equator
• No warm sea water in South Atlantic,
hence no South Atlantic Hurricanes
• No warm sea water in Southeast Pacific,
hence no Southeast Pacific Hurricanes
• Apart from Caribbean coast, no hurricanes
in South America (maybe?)
March 2004: Brazil’s First Hurricane?
Coriolis Effect at Equator
Coriolis Effect at Equator
Coriolis Effect at Equator
• Westbound: Deflected away from Equator
• Eastbound: Directed along Equator
• Unlikely for winds but does happen in
oceans (Equatorial Countercurrent)
• Weather systems can’t spin
Saffir-Simpson Scale
Defined by instruments
1. 74-95 mph
1-2m storm surge
2. 96-110 mph
2-3 m
3. 111-130 mph
3-4 m
4. 131-155 mph
4-6 m
5. >155 mph
>6m
Naming Hurricanes
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No naming system until 1953
Women’s names 1953-79
Regional Name Lists
Lists maintained by World Meteorological
Organization
• Names can be retired after especially
significant storms
Naming Hurricanes
Dangers of Hurricanes
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Wind Pressure
Flying Debris
Storm Surge
Flash Flooding
Tornadoes
Eye of Hurricanes
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100 km or less in diameter
30 minutes or so calm weather
Definitely not the end of the storm!
Post-eye storm is stronger
“Centrifugal” force counteracts inward air
flow
• In strongest storms, air flow can get so
congested a second eyewall forms
(Andrew)
Trailing Side is Most Dangerous
Decay of Hurricanes
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Need warm water for energy
Decay rapidly over land
Lose strength over cold water
Can still cause destructive flooding long
after cyclonic structure is gone
• Degenerate into low pressure systems
Cold Water Trail
Extratropical “Hurricanes”
Two-Ocean Hurricanes
Winter Storms
• Blizzard = Blowing snow and reduced
visibility
• Main Hazards
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Vehicle accidents
Hypothermia
Exertion
Immobility
Great Blizzards
• Schoolhouse Blizzard, Dakotas-Nebraska,
Jan. 12, 1888: 235 killed
• Great Blizzard, East Coast, March 12, 1888:
400 killed, 200 ships sunk
• Armistice Day Blizzard, upper Midwest,
Nov. 11, 1940: 154 killed
• Storm of the Century, March 12, 1993:
Eastern U.S.: 270 died and 48 missing at
sea