Severe Storms

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Severe Storms

• Most Violent weather is associated with low
  pressure systems because air of different
  properties mixes there
• Thunderstorms
• Tornadoes
• Hurricanes
• Winter Storms

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Thunderstorms

• Flash Flooding
• Hail
• Lightning
• Downbursts
• Tornadoes

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Flash Flooding
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Lightning
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Thunder
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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

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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)

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Mammatus
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Squall Line
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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 lt1
• F6? How to identify?

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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

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Mesocyclone
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Mesocyclone, Door County, August 2007
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Door County Tornado, August 1998
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Door County Tornado, August 1998
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Door County Tornado, August 1998
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Door County Tornado, August 1998
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Door County Tornado, August 1998
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Door County Tornado, August 1998
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Door County Tornado, August 1998
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Langlade County Tornado 2007
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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

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Tornado Myths

• 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?

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Fort Worth, Texas, March 28, 2000
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Tornadoes do not avoid cities
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Things Often Mistaken For Tornadoes

• Heavy Precipitation
• Downbursts
• Dust Devils
• Cold Funnels
• If Theres No Evidence of Rotation, Its Not a
  Tornado

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Virga
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Downburst, May 1994
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Downburst Damage, Ontario
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Dust Devil
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Cold Funnels
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Cold Funnels
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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

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World Hurricane Tracks 1995-2003
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Hurricane Forming Regions
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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?)

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March 2004 Brazils First Hurricane?
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Coriolis Effect at Equator
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Coriolis Effect at Equator
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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 cant spin

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Saffir-Simpson Scale

• Defined by instruments
• 74-95 mph 1-2m storm surge
• 96-110 mph 2-3 m
• 111-130 mph 3-4 m
• 131-155 mph 4-6 m
• gt155 mph gt 6 m

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Naming Hurricanes

• No naming system until 1953
• Womens names 1953-79
• Regional Name Lists
• Lists maintained by World Meteorological
  Organization
• Names can be retired after especially significant
  storms

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Naming Hurricanes
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Dangers of Hurricanes

• Wind Pressure
• Flying Debris
• Storm Surge
• Flash Flooding
• Tornadoes

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Eye of Hurricanes

• 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)

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Trailing Side is Most Dangerous
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Decay of Hurricanes

• 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

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Cold Water Trail
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Extratropical Hurricanes
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Two-Ocean Hurricanes
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Winter Storms

• Blizzard Blowing snow and reduced visibility
• Main Hazards
• Vehicle accidents
• Hypothermia
• Exertion
• Immobility

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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