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