Lecture 4: Tornadoes

• Tornadoes


  A violently-rotating column of air that is vertically-oriented and that descends (to ground level) from a thunderstorm cloud (cumulonimbus)
• Tornadoes


  • Often visible due to a condensation funnel in which dust and debris is picked up and rotated
  • Come in different shapes ('wedge', 'rope')
• Waterspouts


  Tornadoes over water
• Tornado formation


  1. Formation of a mesocyclone (a rotating updraft some ~2-6 miles in diameter) within a parent thunderstorm

  2. Tornado is an extension of the larger mesocyclone rotation within the supercell storm
• Thunderstorms


  • Form when warm, moist air rises rapidly ('instability' characterised by warm moist air that is less dense near the surface and cold dry air above)
  • Very severe thunderstorms (bigger, longer-lasting) requires other environmental conditions to be met (e.g. significant vertical wind shear, large instability) and have a complex pattern of updraft and downdrafts
• Supercell thunderstorms


  Often produce large damaging hail and torrential rains
• Tornadoes have been observed on all continents (expect Antarctica)
• Tornadoes are most frequent, most intense and most damaging in the USA (~1200 per year)
• Most tornadoes occur in the Great Plains of central USA where environmental conditions for severe thunderstorms are common
• Tornado seasonality in the USA


  • April
  • May
  • June
  • Oct
  • Nov
  • Dec
• Tornado Alley


  Central USA region that suffers the most numerous and devastating tornadoes (~1000 reported per year, causing ~100 deaths in total)
• Tornado Alley


  • Region is loosely defined but extends from Texas north to Minnesota
  • Tornado season runs from spring to early summer
• Key ingredients for supercell thunderstorms meet in Tornado Alley (conducive to tornado formation)
• Significant trends in the spatial frequency of annual tornado reports have been found (1979–2017)
• Tornadoes elsewhere


  • Tay Bridge train disaster, 1879, caused by a waterspout (75 people killed)
  • Birmingham tornado (July 2005) had windspeeds of 137-186 mph and caused £40 million damage
• Fujita scale


  A tornado intensity scale developed by Fujita (1973) based on tornado-inflicted damage to human-built structures & vegetation and assigned wind speed
• Enhanced Fujita scale (EF-Scale)


  More accurate scale that has been adopted, reflects a tornado's highest attained intensity
• Most tornadoes are weak (EF0 or EF1), but only ~2% are EF4 or EF5 and these account for 60-70% of deaths
• Tri-state Tornado (1925)


  Record-holding tornado: longest duration (3.5 hours), most destructive (~700 killed, 15,000 homes destroyed), longest continuous path (350 km), widest path (up to 1.5 km), $17 million damage (1925 USD)
• Tornado hazards and damage

  • Much of the hazard arises as a consequence of the extreme winds (projectiles, collapsing buildings etc.)
  • Damage can be extremely localised – e.g. total obliteration of one street, while the next street is unscathed
• Reducing tornado vulnerability


  • Construct concrete/underground shelters
  • Educate public and especially most vulnerable communities
  • Optimise detection and forecasting protocols
• Tornado detection

  A multi-faceted approach employed in the USA, with forecasters assimilating data collected from doppler radar technology, from spotters and from satellite
• Tornado Warning System (USA)

  1. Forecast tornadigenic weather conditions

  2. Issue a 'Tornado Watch' alert and intensify monitoring

  3. Try to detect tornadoes using radar stations, spotter networks, satellite images

  4. Issue a 'Tornado Warning' if a tornado is reported on the ground, a funnel cloud has been reported, strong low-level rotation is indicated by weather radar, or a waterspout is headed for landfall

  5. Initiate emergency plans: take refuge!
• SKYWARN Storm Spotter Program


  Part of the National Weather Service (NWS), with ~375k trained local volunteers who act as severe weather spotters
• Tornado detection: spotter networks

  • Spotters look for characteristic cloud formations like mammatus clouds, as well as other warning signs like dark greenish/black sky, large hailstones falling, clouds moving very fast, rushing/roaring sound, debris dropping from the sky
• Tornado detection: satellite images

  • Visible - look for shadow cast by overshooting top of thunderstorm cloud (daytime only)
  • Infrared (temperature information, colour enhanced) – thunderstorm clouds tend to be cooler (higher in atmosphere) than other clouds
  • Moisture – look for adjacent moist (white) and dry (black) air masses
• Doppler Radar


  Emits radio waves, detects the strength of the returned signal, time taken to travel to the object and back (hence distance), and frequency shift of the pulse (Doppler effect) to calculate the speed of the object towards or away from the radar
• What can Doppler radar tell us

  • Reflectivity shows rainfall intensity
  • Velocity shows direction of air movement: green = air is moving "inbound" towards the radar, red = air moving "outbound" away from the radar
  • Hook shape indicates meso-cyclonic (supercell) rotation