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

Thunderstorms 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 F mph29% F mph40% F mph24% F mph6 % F mph2 % F 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

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 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 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

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

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

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 mph1-2m storm surge mph2-3 m mph3-4 m mph4-6 m 5.>155 mph> 6 m

Naming Hurricanes No naming system until 1953 Women’s names Regional Name Lists Lists maintained by World Meteorological Organization Names can be retired after especially significant storms

Naming Hurricanes

Dangers of Hurricanes Wind Pressure Flying Debris Storm Surge Flash Flooding Tornadoes

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)

Trailing Side is Most Dangerous

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

Cold Water Trail

Extratropical “Hurricanes”

Two-Ocean Hurricanes

Winter Storms Blizzard = Blowing snow and reduced visibility Main Hazards –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