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PostPosted: April 27th, 2013, 12:38 pm
by Jessica
General info
  • Generated in regions of already severe thunderstorms
  • Two prime factors for initiation: topography, local climate
  • Created over areas undisturbed by mountains
    • Very common in Midwestern US – “Tornado Alley” (TX, OK, KS, NB, IA)

  • Average dimensions: width – 75 yards; track length – 1-4 miles; time on ground – 5 min; forward speed – 25 mph
  • Timing – spring to summer; late afternoon to late evening (time of maximum local ground heating)
  • Structure
    • Combination of opposing wind patterns
      • High level, relatively dry/cooler winds from west
      • Lower level, warm/wet winds from east (originating in the Gulf of Mexico)
    • Rollers – winds form cyclonic rotation parallel to the ground
    • Large thunderstorms pull warmer air upward from low levels; warm/wet air provides energy for storm
    • Large updrafts can cause a roller to tip into a vertical position
    • Tornado is born upon touchdown with ground
    • Large storms can produce more than one tornado – called “families” or “swams”
      • Large tornado swarm in 1974; caused damage in 13 states from AL to OH; 148 tornadoes in ~16 hours; 307 killed, 6,000+ injured and $600 mil in damage
  • Severe pressure drops
    • Inside a twister can be as low as 3″ Hg
    • For comparison: average daily pressure is ~31-32″ Hg; large thunderstorm can be as low as 27″ Hg
  • Movement
    • Track direction generally SW to NE
    • Combination of prevailing wind direction from west and the counterclockwise rotation of tornado