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Announcements Announcements • No office hours tomorrow (Wed. March 30) • Homework 5 extra credit: – Find and plot additional tracks of Hurricane Ivan (after reaching its northeastmost point in U.S) Lightning and Tornadoes North Dakota Outline • Finish lightning/thunderstorms • Tornado Facts • Formation and Conditions Needed • Damage Levels • U.S. Occurrences • Examples Electrification of Clouds • Generally upper level of clouds - + charge • Mid level - negative charge • Lower level - mix of charge • Why? Transfer of positive ions from warm to cold objects in collisions between ice droplets, water droplets Upper level + Mid level - Lower level mix Note positive charge on high objects near cloud negative charges Lightning Strike • Unlike charges attract each other – Negative charges in cloud lead to positive charges on ground – Dense on high objects (trees, poles) • Set up electric potential between cloud and ground • When large enough, current flows - lightning Lightning Facts • Several strokes occur over ~1/2 second • Travels over 6,000 miles/second • Can produce very high temperatures (55,000 F) briefly – High temps cause air to expand, produce sound wave (thunder) Deaths • ~143 deaths per year in U.S. • Ways to avoid – Stay inside house, car, truck – Outside, move to low place Winds • Can be straight-line or rotating (tornadoes) • Straight-line winds can be 80-100 mph – Damage structures, trees – Duration can be hours What are tornadoes? • Rapidly rotating winds usually descending from thunderstorm • Most violent storms with highest windspeeds • Average U.S. fatalities ~100/year • Common in Great Plains Tornado Facts • Typically ~100-600 m wide – Some up to 1 mile wide • Speeds – Travel up to 62 mph (100 km/hr) – Wind speeds up to 300 mph (rare, but most deadly) Formation • Some details still unknown • Tend to form with severe thunderstorms • Appear to require additional component to cause shearing of thunderstorm cloud Conditions • Some same as needed for hailstorms – Northward flow of warm, moist air (Gulf of Mexico) – Cold dry air from Canada or Western U.S. • Jet stream - key component for spinning the cloud – High enough that it starts top of cloud spinning, rest of cloud pushed by mid-level air motion Key Components: Warm moist air Cold dry air Jet stream Differences between Systems • Single cell – Updrafts of warm air, downdrafts (rain) from condensing, cooling air Differences between Systems • Supercell thunderstorm – Updrafts on leading side: rain – Downdrafts on trailing side: rotating air packets that can form funnel clouds (tornadoes after hit ground) – Many have centers with rapidly upflowing air (sucks up material) Importance of Rotation • Not Coriolis force issue (feature too small to be affected) • Once rotation starts, pulls into tight spiral – Speed increase – Smaller diameter, faster it spins Structure of tornado • Can be horizontal or vertical • Winds vary within cloud – Highest a few 100 ft above ground • Objects (trees, buildings, etc) cause friction, slows wind speed close to surface Destruction from Tornadoes • High speed winds – Damage to trees, buildings, etc • Lifting force into funnel – Vehicles, people, other loosely attached objects • Explosions due to extreme changes in air pressure – Very low pressures inside funnel, higher pressures outside the funnel Measuring Damage • Fujita Scale – Developed in 1060s – Based on wind speed damage Fujita Scale Category F0: Light Damage (<73 mph); Some damage to chimneys; branches broken off trees; shallow- rooted trees pushed over; sign boards damaged. Category F1: Moderate Damage (73-112 mph); Peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos blown off road. Category F2: Considerable Damage (113-157 mph); Roofs torn off frame houses; mobile homes demolished; boxcars overturned; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground. Category F3: Severe Damage (158- 206 mph); Roofs and some walls torn off well-constructed houses, trains overturned; most trees in forest uprooted; heavy cars lifted off ground and thrown. Category F4: Devastating Damage (207- 260 mph); Well-constructed houses leveled; structure with weak foundations blown off some distance; cars thrown and large missiles generated. Category F5: Incredible Damage (261- 318 mph); Strong frame houses lifted off foundations and swept away; automobile sized missiles fly through the air in excess of 100 meters (109 yards); trees debarked; incredible phenomena will occur. U.S. Occurrences • For given year (1992) there were – 696 F-0 – 411 F-1 – 129 F-2 – 43 F-3 – 13 F-4 – 1 F-5 ** Note that these are very rare events Where and When are Tornadoes Common in the U.S.? • Where: interior of U.S. is tornado capital of the world – Why? Conditions there are great for formation Thunderstorms Tornado map doesn’t quite match the thunderstorm occurrence map, but is close to the hailstorm map. Why? hailstorms Need cold dry air! Deaths per state, 1950-1994 Where and When are Tornadoes Common in the U.S.? • When: late spring, early summer – Varies, peak moves north during summer – Why? Conditions right during those months (key jet stream positioning) Fatalities • Common locations – Mobile homes • No interior rooms for protection • Weaker coupling to ground – Exterior rooms with windows • Not as much protection as solid walls Best Places to Ride out a Tornado • Basement • Interior closet, bathroom, hallway on lowest floor of a building • If outside, find low point (ditch, streambed) Predictions and Warnings • # of deaths/decade is decreasing Why? Prediction and Warning • Less fatalities partially from building design • Partially from development of warning systems – Tornado watch: conditions are right for events – Spotters, weather radar look for tornadoes – Warning issued after touchdown spotted • Sirens • Radio/TV broadcasts • Can provide minutes of warning Tri-State Tornado • March 1925 • Largest known single tornado (F-5) • Lasted over 3.5 hours • Traveled 219 miles through MO, IL, IN • Swath of ~1 mile wide (widest recorded) • 689 fatalities Murphysboro: most fatalities Why so many fatalities? No warning system 1974 Super-Outbreak • Weather conditions collide – Cold front from Rockies – Low pressure moving east – Humid air over Gulf of Mexico – Polar jet stream down into TX – Dry air mass from SW moving towards low pressure • Form inversion layer: dry air above moist air 1974 Super-Outbreak • Moist air breaks through to form huge thunderclouds • Set spinning by jet stream, converging air masses • 16 hours, 147 tornadoes in 13 states – 6 were F-5 tornadoes – 24 were F-4 F-5 Ohio F-4 Indiana Ohio subdivision after 1974 Outbreak Recent • Tornado Activity in 2005 (from National Weather Service) • Jan 13, 2005 The 2005 tornado season got off to an early start. A destructive F3 tornado moved across 20 miles of Union County, Arkansas from Junction City to Lawson. Two people were killed, both 83 years old, in separate mobile homes about a mile apart. About 30 homes were destroyed. • January 13, 2005 In a different storm, a married couple died at Arlington, Early County, Georgia, when their mobile home was destroyed. Only in rural areas? • NO! • Can form in cities – Although they are small targets • Recent (late 1990’s-2000) events include – F-5 in Oklahoma City (42 fatalities), F-4 in Wichita, KS; Cincinnati, OH (6-7 fatalities each) Outbreak in Oklahoma had ~59 tornadoes Remote sensing satellite data shows swath hit by tornado near OK City Waterspout • Rotating column of air over large body of water – Can be a tornado that formed over land and then traveled over water – Some form over water: “fair weather” waterspouts • Generally smaller diameter than average tornado, move more slowly, shorter duration Next Time • Hurricanes Part 1.
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