Tornadoes

[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)

Formation

• 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