Tornados

General Info

  • Generated in regions of already severe thunderstorms 
  • Two prime factors for tornado initiation 
    • The general topography 
    • The local climate 
  • Created over areas where air is undisturbed by mountains 
    • Very common in midwest US 
    • Known as tornado alley (tx, ok, ks, nb, ia, mo, il)
    • Most rare in alaska, texas has the most 
    • Average by area though is florida 
  • Is tornado alley changing with the changing climate 
    • Gradually shifting eastward

Tornado Formation  

  • Average dimensions 
    • Width: 75 yrds 
    • Track length: 1-4 miles (how long its on the ground)
    • Time on ground: 5 minutes 
    • Forward speed: 25 mph
  • Timing 
    • Prime season for the combination of ideal conditions 
    • Spring to early summer 
    • Although this appears to be shifting earlier 
    • Prime time of day
    • Late afternoon to late evening 
      • Time of maximum local ground and atmosphseric heating 
      • The hottest part of the day
  • Tornado structure
    • Combinations of opposing wind patterns 
    • High level, relatively dry/cooler winds from the west 
    • Lower level, warm/wet winds from the southeast originating in the golf of mexico 
  • Windows from cyclonic rotation parallel to the ground (called rollers)
    • Large thunderstorms pull warmer air upward from low levels 
    • warm/wet air provides energy for the storm
  • Formation 
    • Large updrafts can cause a roller to tip into a vertical position 
    • Tornado is born upon touchdown with the ground
    • Large storms can produce more than one tornado
    • Called tornado families or storms 
    • Ex - large tornado swarm in 1974 caused damage to 13 states from AL to OH
      • 148 tornados touched down in about 16 hours 
      • 307 killed 
    • Severe pressure drops (measured on a barometer) 
    • Inside a twister can be as low as 3” mercury 
    • For comparison 
      • Average daily pressure her is 31-32”
    • Movement 
    • Track direction is generally SW to NE 
    • Combination of the prevailing wind direction from the west 
    • The CCW rotation of the tornado
  • Fujita scale
    • Varies from EF0 to EF5 
    • EF0 - weakest, with winds of 65-85 mph and minor damage to building/trees
    • EF5 - strongest with winds of >200 mph and complete destruction 
    • Precise wind speed numbers are actually estimates based on damage after the storm and not measured directly during 
    • EF0 = light damage some damage to chimneys; branches broken off trees; shallow-rooted trees pushed over; sign boards damaged
    • EF1 = Moderate damage. Peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos blown off roads
    • EF2 = Considerable damage. Roofs torn off frame houses; mobile homes demolished; train cars overturned; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground
    • EF3 = Severe damage. Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off the ground and thrown.
    • EF4 = Devastating damage. Well-constructed houses leveled; structures with weak foundations blown away some distance; cars thrown and large missiles generated.
    •  EF5 = Incredible damage. Strong frame houses leveled off foundations and swept away; automobile-sized missiles fly through the air in excess of 100 meters (109 yds.); trees debarked

Monitoring 

  • Early 1980s 
    • Device called TOTO
    • Designed to get swept up into a tornado and measure all the atmospheric parameters of a tornado from the inside
    • Scientists had to race to get ahead of a tornado and place the 400 pound cylinder squarely in its path
    • Never actually worked
  • Storm prediction center 
    • Located in norman OK
    • Monitors regional weather conditions every morning 
    • Issues a risk warning for severe weather later that day 
    • Interacts with national weather service 
  • National severe storms lab
    • Use of doppler radar 
    • Measures the frequency change of objects moving away from the antenna 
    • Interpets mesocyclones 
  • Mobile doppler radar
    • Operated on a mobile platform to image nearby tornadic activity 
    • Can get much closer to the tornado to image it more clearly 

Mitigation 

  • No practical efforts 
    • Most injuries/death occur from flying objects or being picked up and thrown around 
  • Scientific instrumentation and early warning systems are being improved 
    • Detection lead times are only a matter of minutes to no more than an house 
    • Very localized tracking by local tv 
  • Safe rooms lined with concrete