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Tornadoes: Terminology
 Tornado: a violently rotating column of air in
contact with the ground. Also called “twister”
and “cyclone”
 Waterspout: a tornado over water, generally
much weaker than a tornado
 Funnel Cloud: a rapidly rotating column of air
not in contact with the ground, often the
precursor to a tornado, extending from the
base of a cumulonimbus cloud
Tornado Safety
 Avoid blowing debris (don’t become a part of it,
either)
– Get underground, if possible (wind speed = 0)
– Find a ditch or low spot and cover your head
– Interior, windowless room of a sturdy building,
preferably under something that won’t fall on you
but will protect you from falling/flying debris
– Avoid cars and mobile homes, highway overpasses
Tornado
Intensity
 The Fujita Scale: based upon damage surveys
and not direct measurement of winds
– Similar to Saffir-Simpson scale for hurricanes
– Ranges from F0 (EF0) for very weak tornadoes to
F5 (EF5) for incredibly strong tornadoes
– Scale goes higher (F12) but it’s unlikely we’ll see
anything stronger than an F5- how can damage be
any worse than “incredible?”
– The strongest tornadoes develop within supercells
– Enhanced Fujita Scale
Supercell Thunderstorms
 Produce the most extreme
severe weather
– Large, strong tornadoes
 Wind shear- direction, speed
 Rotating thunderstorms
– Long (~6 hour) lifespan
 Wind shear- direction, speed
 Tilted updraft
– Large hail and damaging winds
 Extreme instability
 Extremely strong updrafts and
downdrafts
Supercells: Extreme Vertical Motion
 The Lid (capping inversion)
– Warm, dry air overlies warm, moist air
– Result of low-level S,SE winds from Gulf of Mexico
and SW winds above this layer blowing from the
Mexican plateau
– Limits development of “wimpy” thunderstorms: more
energy available for a few supercells versus lots of
run-of-the-mill thunderstorms
Supercells: Extreme Vertical Motion
– Breaking the Lid
 Daytime heating weakens it
 Rising air will cause a more rapid cooling aloft than near
surface: result is explosive thunderstorm development
Supercells: Long-Lived Monsters
 Directional and
speed shear tilts
the updraft
 Separates inflow
and outflow
– Constant supply of
warm, moist air
Supercells: Tornado Development
 Directional shear induces thunderstorm rotation
 Speed shear creates horizontal roll
 Roll tilts vertically in updraft
Supercells: Tornado Development
 Strong updrafts and downdrafts, especially the
rear flank downdraft, appear to be important in
tornadogenesis
– From Markowski et al,
2002, Direct Surface
Thermodynamic Observations
within the Rear-Flank
Downdrafts of Nontornadic
and Tornadic Supercells,
Monthly Weather Review
Tornadoes
 Suction Vortices: think of them as mini
tornadoes within the primary funnel
Image from www.atmo.arizona.edu/.../apr2609.jpg
 Are tornadoes becoming more frequent?
Key Figures
 Tables 14.8, 14.9, 15.8, 15.10, 15.12
 Interesting Time-Lapse Movie of
Thunderstorm Development
 Time-Lapse 2 / Time-Lapse 3
 Multi-Vortex Tornado
 M-V Tornado from Helicopter
 Rotating Supercells in Wyoming/Texas
 What you normally should NOT do