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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. Flash Flooding Hail Lightning Downbursts Tornadoes. Flash Flooding. Lightning. Thunder.

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Severe Storms

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

  2. Thunderstorms • Flash Flooding • Hail • Lightning • Downbursts • Tornadoes

  3. Flash Flooding

  4. Lightning

  5. Thunder

  6. 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) • Notone second = one mile • “Heat Lightning” is ordinary lightning illuminating the clouds

  7. 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)

  8. Mammatus

  9. Squall Line

  10. 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?

  11. 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

  12. Mesocyclone

  13. Mesocyclone, Door County, August 2007

  14. Door County Tornado, August 1998

  15. Door County Tornado, August 1998

  16. Door County Tornado, August 1998

  17. Door County Tornado, August 1998

  18. Door County Tornado, August 1998

  19. Door County Tornado, August 1998

  20. Door County Tornado, August 1998

  21. Langlade County Tornado 2007

  22. 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

  23. 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?

  24. Fort Worth, Texas, March 28, 2000

  25. Tornadoes do not avoid cities

  26. Things Often Mistaken For Tornadoes • Heavy Precipitation • Downbursts • Dust Devils • Cold Funnels • If There’s No Evidence of Rotation, It’s Not a Tornado

  27. Virga

  28. Downburst, May 1994

  29. Downburst Damage, Ontario

  30. Dust Devil

  31. Cold Funnels

  32. Cold Funnels

  33. 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

  34. World Hurricane Tracks 1995-2003

  35. Hurricane Forming Regions

  36. 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?)

  37. March 2004: Brazil’s First Hurricane?

  38. Coriolis Effect at Equator

  39. Coriolis Effect at Equator

  40. 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

  41. 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 • >155 mph > 6 m

  42. Naming Hurricanes • 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

  43. Naming Hurricanes

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

  45. 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)

  46. Trailing Side is Most Dangerous

  47. 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

  48. Cold Water Trail

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