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Tornadoes Chapter 14B Tornadoes • Tornado - rapidly rotating column of air that blows around a small area of intense low pressure with a circulation that reaches the ground. • Funnel cloud- tornado beginning to form but has not reached the ground. 30% of funnel clouds become tornadoes https://www.youtube.com/watch?v=7KDz6dGQ 5RE Tornadoes – Most rotate counterclockwise around low pressure in North America, with wind speeds less than 100 knots (115mph), – Can have speeds above 220 knots (253mph) – https://www.youtube.com/watch?v= cvOut9VUqKY Tornadoes • Tornado life cycle – Dust-whirl stage- dust swirling upward from the surface marks the tornado’s circulation on the ground, short funnel from thunderstorm • damage is light – Organizing stage- tornado increases in intensity with downward extend of funnel – Mature stage – funnel reaches greatest width and is almost vertical • damage is most severe Tornadoes • Tornado life cycle – Shrinking stage – decrease in funnel width and increase in tilt, narrowing of damage at the surface (can still have intense damage) – Decay stage – tornado shaped like a rope, twisted/contorted, then dissipates. • Tornadoes may not necessarily reach all stages and some can skip stages. • After mature stage, usually stays in contact with ground until it dissipates. Tornadoes • Tornado Occurrence – US experiences the most tornadoes in the world, 1000 annually. Can occur in all states, even Hawaii and Alaska. – Most occur in Tornado Alley, which stretches from central Texas to Nebraska. – Central Plains has warm, humid surface air and cold dry air aloft. Large supercell thunderstorms can form with strong wind shear that produces tornadoes – Highest in spring, lowest in winter, because of warm, humid surface air. Most frequent in late afternoon because air is most unstable. Tornado Alley, with highest incidence of tornadoes. Figure14-43 p407 Tornadoes • Tornado occurrence locations – Depends on polar jet stream – In winter, jet stream is above southern Gulf states, where contrast between warm and cold air masses is greatest – In spring, humid Gulf air and jet stream moves northward toward Great Plains. – In summer, jet stream is near Canadian border, more tornado activity northern plains and New York Tornadoes form due to cold, dry air meeting warm, humid air, due to polar jet stream. As the jet stream moves with seasons, the locations of tornadoes move with it. Tornado incidence by month. Average number of tornadoes during each month in the United States from 2000 to 2010. (NOAA) Figure14-44 p408 Tornadoes • Tornado winds – Measurement based upon damage after storm or Doppler radar – Most studies have shown that severe tornadoes have 220 knots maximum speed, with most having less than 125 knots. Tornadoes – Multi-vortex tornados have smaller whirls rotating within, called suction vortices that are only 30ft in diameter but are very fast and can cause great damage. Figure14-45 p408 The total wind speed of a tornado is greater on one side than on the other. When facing an onrushing tornado, the strongest winds will be on your left side. Figure14-46 p409 Tornadoes • Seeking shelter – Basement or small, interior room on ground floor, mattress can reduce injury from flying debris. – Tornado has very low pressure at center, which can pull the roof and walls off of a structure. – Windows are often shattered which increases pressure. – If outside, lie in a ditch or ravine. Tornadoes • Tornado watch- tornado is likely to form in the next few hours. Often there are trained volunteer spotters who look for tornadoes. • Tornado warning- tornado is spotted, visually or on radar screen. • Tornado sirens or radio and television warnings can alert population. Fujita Scale • Enhanced Fujita Scale – Based upon the wind speeds and consequent damage of storm – EF0 weakest, EF5 strongest – 1-2 EF5s annually in U.S. Table 14-3 p411 Tornadoes – Example of an EF5 tornado in Joplin, Missouri on May 22, 2011, injured 1000 people and killed 159, the greatest death toll from a single tornado since 1947. – Today show coverage of Joplin tornado: https://www.youtube.co m/watch?v=vn55m56tQ 6s Tornadoes • Yearly average of tornado deaths is less than 100, although over 100 can die in a single day, as in Joplin in 2011. • 45% of all fatalities occur in mobile homes. • Deadliest tornadoes occur in families, when different tornadoes are spawned by the same thunderstorm. • When 6 or more tornadoes occur over a particular region, it is termed a tornado outbreak. Tornado Statistics • Super outbreak- many tornadoes in a short period of time. • One of the most violent super outbreaks occurred on April 3-4, 1974 in the U.S. • During a 16-hour period, 148 tornadoes cut through 13 states, killing 307 people, injuring 6000, and causing $600m in damage. • Thirty were F4-F5 (six were F5), with a total destruction path of 4181 km. • From this outbreak, severe weather safety training in schools began to be implemented across the U.S., and the government began a ‘natural disaster reduction initiative’ to increase preparedness and awareness. Radar technologies like Doppler radar were not around at the time. Tornado Statistics • Deadliest tornado was March 18, 1925 when 695 people died in Missouri, Illinois, and Indiana. • More recent super outbreak was in 2011 across 3 days where 336 tornadoes broke out across the southern U.S., hardest hit was Tuscaloosa, Alabama. • Despite new technology like Doppler Radar and improved warning systems since 1974 super outbreak, tornadoes can still be very deadly. Tornado Formation • Basic requirements for tornado formation are an intense thunderstorm, conditional instability, and strong vertical wind shear. • Supercell Tornadoes – Supercell thunderstorm: single rotating updraft that can last for hours. – Warm humid air is drawn in and spins counterclockwise as it rises, mixes with dry air and some evaporates and cools and makes a downdraft, called a forward-flank downdraft. Supercell Tornado Formation • Wind shear causes spinning vortex tube that is pulled into thunderstorm by the updraft; Supercell Tornado Formation • The tube can be tilted from horizontal to vertical. Supercell Tornado Formation • The rising, spinning air is called a mesocyclone, which lowers the pressure in the mid-level of the thunderstorm. It can be 5-10km across. • Precipitation can also be spun inside counterclockwise around the updraft, which can be seen by radar, although there is no precipitation in the mesocyclone. • As the precipitation is drawn into a spiral around the mesocyclone, it can look like a hook, called a hook echo. Figure14-54 p414 Tornado Formation • Tornadogenesis- the formation of a tornado • As the rain-chilled air of the rear-flanked downdraft wraps around the updraft, there is additional spin in the mesocyclone, and the updraft at the base rises more slowly than the updraft aloft. Supercell Tornado Formation • The rising updraft stretches vertically and thins horizontally, which causes the air to spin faster (centripetal forcelike the skater pulling her arms in spins faster). • If it keeps stretching, it can shrink into a narrow column of rapidly rising air – a tornado vortex. Supercell Tornado Formation • As the air spins around the low-pressure core of the vortex, the air expands, cools, and condenses into a visible cloud- a funnel cloud. • As the air is drawn into the core, the air cools and condenses and the funnel cloud descends. • The cloud can then reach the ground and pull in dirt and debris. Supercell Tornado Formation • The air outside the tornado is spiraling upward, but the air inside the tornado descends down and warms, causing evaporation. RFD: rear-flank downdraft FFD: forward-flank downdraft Tornado Formation • Supercell Tornadoes – Less than 15% of supercells produce tornadoes. – First sign of tornado is the sight of rotating clouds. – The rotating cloud can lower and become a wall cloud. – Sometimes the air is dry so the funnel remains invisible until it hits the ground and picks up dust. – Some tornadoes sound like a freight train, while others can be silent. Figure14-55 p415 Non-supercell Tornadoes • Nonsupercell tornadoes – Tornadoes not associated with a pre-existing wall cloud in a supercell are called nonsupercell tornadoes. – Some extend from a thunderstorm and others can build upward from the ground. – When formed along a gust front it is called a gustnado. – Waterspouts are connected to a cumuliform cloud over a large body of water. – Weak, short-lived tornadoes that rapidly build with cumulous congestus clouds are landspouts. Figure14-57 p418 Figure14-58 p418 Figure14-59 p418 Observing Tornadoes and Severe Weather • Doppler radar measures the speed of precipitation toward and away radar unit – Mesocyclones and tornadoes have a unique image on the radar display- a tornado vortex signature. • NEXRAD – 150 Doppler radar units within the continental U.S. – Algorithms to give warning of tornado. Figure14.60 Doppler radar display of winds. The close packing of the horizontal winds blowing toward the radar (green and blue shades), and those blowing away from the radar (yellow and red shades), indicate strong cyclonic rotation and the presence of a tornado. Figure14-60 p419 Figure14.61 Doppler radar display showing precipitation inside a large supercell that takes on the shape of a hook. Figure14-61 p419