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Meteorology Unit 5 - Tornadoes What is a tornado? • Tornado – a violently destructive windstorm, characterized by a long, funnel-shaped cloud extending toward the ground and made visible by condensation and debris • Almost all tornadoes rotate counterclockwise around a central core of low pressure • A few have been seen rotating clockwise, but this is extremely rare Tornado AKA • Also called twisters or cyclones • Funnel cloud – tornado that has not reached the ground • Waterspout – a rotating column of air over a body of water Tornado Structure • Diameter between 300 – 2,000 feet • Few have been over 1 mile wide – 2004, Nebraska – 2.5 miles wide • Usually travel 20-50 mph, some clocked at 70+ mph • Usually only last a few minutes and have an average path length of 4 miles – 1917, tornado traveled 292 miles through Illinois and Indiana over a 7 hours Tornado Life Cycle • Dust-whirl Stage – dust swirling up from the ground marks the tornadoes circulation and a short funnel extends from thunderstorm’s base • Organizing Stage – tornado increases in intensity, funnel extends downward • Mature Stage – Funnel reaches its greatest width and contacts ground. Vertical. This is the stage where the most damage occurs • Shrinking Stage – decrease in funnel width, narrowing in path, tornado may tilt. Still can cause severe damage. • Decay Stage – tornado looks rope-like and eventually dissipates Tornado Formation • Form with intense thunderstorms and conditionally unstable environments • Usually with supercell storms with strong wind shear • Rotating air may begin in the storm and work downward or begin on the ground and work upwards • Fueled by convective circulation of warm updrafts and cold downdrafts Vortex Tube • Strong updrafts form a vortex tube. If updrafts tilt and pull the vortex tube into the storm, it becomes a rotating air column within the storm Vortex Tube Tornado Occurrence • Occur in many parts of the world, but most common in the United States – USA averages more than 1,000 yearly – Record 1722 tornadoes in 2004 • Greatest number occur in Tornado Alley in central US • Tornado Alley often provides the right atmospheric conditions Tornado Occurrence • Most tornadoes occur between March and July • Greatest number occur in May, but the strongest tornadoes usually occur in April • Most frequent between 4:00-6:00pm Tornado Measurement • Fujita Scale – original scale, wind speed was estimated based on damage to surface • Enhanced Fujita Scale (EF Scale) – using 28 different criteria, still uses damage to determine wind speed – – – – – – EF0: 65-85mph EF1: 86-110mph EF2: 111-135mph EF3:136 – 165mph EF4: 166-200mph EF5: >200mph • The majority of tornadoes are weak at EF0 or EF1. The United usually has one or two EF5 tornadoes each year Strange Occurrences • People, animals, and machinery picked up, carried, and (mostly) safely deposited miles away – Railroad coach with 117 passengers was picked up and moved 80 feet – Schoolhouse was demolished and the 85 students inside were carried 300+ feet with no fatalities – Showers of fish or frogs have fallen over land after tornadoes took them from water • Chickens losing all their feathers • A piece of straw driven through a metal pipe • Frozen hot dogs being driven though concrete walls What do I do? • Seek shelter, preferably in a basement or interior room • Stay away from windows • Find a shield, like a mattress • If outside, lay flat in a depression or ravine and cover your head • If in a vehicle, do not try to outrun the tornado. Stop and try to move perpendicular to the tornado. • Listen to the radio/TV for tornado watches and warnings Tornado Myths • Opening windows will keep the house from exploding – Was once thought to help equalize pressure – Now known it actually increases pressure on the opposite wall and increases chance the building will collapse – Damage is mostly caused by wind and debris, not pressure • Take shelter under a highway overpass or bridge – Winds are actually funneled and strengthened by the overpass/bridge structure