Presentation on theme: "Tornadoes A tornado is a violently rotating column of air (vortex) in contact with the ground and a cumulonimbus cloud Tornadoes are capable of inflicting."— Presentation transcript:
Tornadoes A tornado is a violently rotating column of air (vortex) in contact with the ground and a cumulonimbus cloud Tornadoes are capable of inflicting extreme damage
About 7 in 10 tornadoes are weak, with rotating wind speeds no greater than about 110 MPH Less than 2 percent of all tornadoes reach the 200+ MPH incredible category Most violent tornadoes produce home-leveling damage within a very small area…less than 5 percent of the 5,000 affected homes in Wichita Falls, Texas were leveled by this massive 1979 tornado
Winds can exceed 300 mph!!! Can have a path up to a mile wide!!! Around 1,000 occur on average each year in the U.S Kill 80, injure 1,500 people each year on average Can occur any time of the year, but peak during the spring (March-June) 99% of all tornadoes in Northern Hemisphere rotate counterclockwise Texas is #1 for frequency of tornadoes per year Occur most frequently in the central U.S. in a region nicknamed Tornado Alley
How Do They Form? Tornadoes form in a rotating thunderstorm called a super cell. Super cells form when a cold polar air mass meets a warm tropical air mass. The result is a great instability caused by the rising warm air. A narrow zone of cumulonimbus clouds form giving the possibility for tornadoes
Step 1 When thunderstorms develop, an increase in wind speed and/or a large change in direction with height ("wind shear") produces a horizontal, spinning area of air
Step 2 Strong updrafts within the thunderstorm draw this area of rotation up from horizontal to vertical
Step 3 Vertical rotation extends 2-6 miles up into the thunderstorm. Towards the end of this area of rotation (the mesocyclone) is often a lower area of rain-free cloud and can be seen as a rotating "wall cloud"
Step 4 If the rotation intensifies, a funnel cloud can develop where the cloud water vapor is draw down towards the ground The funnel cloud follows the intensity of the vortex towards the ground and touch down completes the formation of a tornado
How Strong Are Tornadoes? EF0 = Weakest, EF5 = Strongest
The Tri-State Tornado Outbreak of March 18, 1925 killed 689 people in Missouri, Illinois and Indiana. Murphrysboro, Ill., had 234 of those deaths, and West Frankfort, Ill., had 127 Other deadly tornadoes include the May 6, 1840 tornado which killed 317 people in Natchez, Mississippi; the May 27, 1896, tornado which killed 255 in St. Louis, Missouri. Tornadoes on successive days in 1936 killed 216 people in Tupelo, Mississippi, on April 5; and 203 people in Gainesville, Georgia, on April 6
Biggest, Costliest Outbreaks The April 3-4, 1974 Super Outbreak was the largest known outbreak, with 148 tornadoes in 11 states, killing 315 people, injuring more than 5,300 and causing $600 million in damages. Alabama, Kentucky and Ohio were the states hardest hit. Perhaps the most notable tornado of the outbreak was one which touched down southwest of Xenia, Ohio. The violent tornado destroyed half the town, killing 34 and causing property damages of more than $100 million
Tornado Myths A highway overpass is a safe place to take shelter during a tornado…when a tornado passes over an overpass, the wind is funneled under the bridge and actually increases in velocity (venturi effect) Opening windows during a tornado balances the pressure between the inside and outside of a house and may prevent destruction…though tornadoes are caused by intense pressure, merely opening windows will not alleviate or equalize this. One should seek shelter in the southwest corner of a house…tornadoes approach from any direction and send flying debris throughout the house. Go to a basement!