Hazards Presentation

Tornadoes

By Tony Brown

IntroductionTornadoes are one of nature's most violent storms. In an average year, about 1,000 tornadoes are reported across the United States, resulting in 80 deaths and over 1,500 injuries. A tornado is a violently rotating column of air extending from a thunderstorm to the ground. The most violent tornadoes are capable of tremendous destruction with wind speeds of 250 mph or more. Damage paths can be in excess of one mile wide and 50 miles long.                                                             

How do Tornadoes form?Tornadoes occur within a variety of different types of storms. The largest and most destructive stem from a long, rotating thunderstorm, or supercell. Within a supercell storm the force of the upcurrent is so strong that the air sweeping into the anvil goes over the upper part of the upcurrents and is cleared of the downcurrents.When the upcurrents and downcurrents separate, a stable convective cell is formed, with warm air rising in the upcurrents and cool air sinking in the downcurrents. This is what occurs within a supercell.A mesocyclone, the speed with which a large volume of air races up the main core of the upcurrent, creates intensely low pressure beneath the cloud. This draws in more air, fueling the low, and results in gale-force winds.

                                                  

Tornadoes can also be found along squall lines as well. A squall line is a line of thunderstorms which can be several miles thick, hundreds of miles long and more than 30,000 feet tall. Squall lines are formed in the same way thunderstorms are created and develop strong upcurrents and downcurrents.The wind during these storms, near the cloud tops, blow from a different direction than the lower-level winds. This change in wind direction is responsible for dispersing high-level air which in turn sets the lower-levels spinning.

                                                            

At the base of a mesocyclone or supercell storm, rotation occurs which makes fragments of cloud turn. As the speed increases, part of the cloud falls below the base of the main cloud. Tornadoes can form under this type of descended cloud, called a wall cloud.A wall cloud is a cyclonically rotating air pattern in which the lower part of the cloud, with no precipitation present, takes humid air into the thunderstorm's main updraft. The wall cloud looks like a solid color cloud attached to the bottom of the main cloud and is an indication of possible tornado formation since it is a lowering of the cumulonimbus cloudbase.

The funnel cloud of a tornado consists of moist air. As the funnel descends the water vapor within it condenses into liquid droplets. The liquid droplets are identical to cloud droplets yet are not considered part of the cloud since they form within the funnel. The descending funnel is made visible because of the water droplets. The funnel takes on the color of the cloud droplets, which is white.As the funnel begins its descent the air on the ground is already rotating because it is being drawn into the upcurrent. Due to the air movement, dust and debris will begin rotating, often becoming several feet high and hundreds of yards wide. Because there is so much energy associated with the moving air it produces a great deal of noise, almost like the roaring sound of a freight train. Tornadic damage results from the force of its winds, yet it is the violent upcurrent and difference in pressure which lifts the large objects in its path.

                                                    

ClassificationTornadoes can be put into three catagories. Weak, Strong and Violent. Weak tornadoes account for 69 percent of all tornadoes and are defined as a tornado with winds under 110 mph. Weak tornadoes last an average of 6 to 10 minutes and account for approximately 3 percent of all tornado deaths. Strong tornadoes account for 29 percent of all tornadoes and approximately 27 percent of all tornado deaths. Wind speeds range from 110 to 205 mph, and these tornadoes may last 20 minutes or longer. Violent tornadoes account for only 2 percent of all tornadoes but result in 70 percent of all tornado deaths. Wind speeds exceed 205 mph, and lifetimes can exceed 1 hour.

Intensity Wind Speed Damage

F0 40-72 mph Light

F1 73-112 mph Moderate

F2 113-157 mph Considerable

F3 158-206 mph Severe

F4 207-260 mph Devastating

F5 261-318 mph Incredible

F-SCALEWINDSCHARACTER OF DAMAGERELATIVE FREQUENCY

F0 (weak)40-72 mph

LIGHT DAMAGE: Some damage to chimneys, TV antennas, roof shingles displaced. Small branches broken on trees.29%

F1 (weak)73-112 mph

MODERATE DAMAGE: Roof decking removed, carports overturned, some trees uprooted, automobiles overturned. Unanchored homes sliding.

40%F2 (strong)

113-157 mphCONSIDERABLE DAMAGE: Roofs blown off homes leaving strong walls standing. Sheds and other outbuildings demolished,

unanchored mobile homes overturned, block structure walls collapsed, roofs peeled back. Small wood missiles observed.24%

F3 (strong)158-206 mph

SEVERE DAMAGE: Exterior walls and roofs blown off homes. Metal buildings collapsed or are severely damaged. Forests are flattened. Most block structures collapsed.

6%F4 (violent)

207-260 mphDEVASTATING DAMAGE: Few walls, if any, standing in well-built homes. Pile of debris on foundation, large steel and

concrete missiles thrown far distances.2%

F5 (violent)261-318 mph

(RARE) INCREDIBLE DAMAGE: Homes on slabs levelled with debris removed. Schools, motels and other marginally engineered buildings have considerable damage with exterior walls and roofs gone. Top stories demolished.

less than 1%

Where are Tornadoes Found?Tornadoes can occur anywhere in the world but are most likely in the United States, east of the Rocky Mountains, during the spring and summer months

  In the southern states, tornado frequency peaks in March through May; while in the northern states, peak frequency is during the summer months. Along the gulf coast, a secondary tornado maximum occurs during the fall. In the western states, the total number of tornadoes is higher than indicated. Sparse population reduces the number reported.

  The average tornado moves from the southwest to northeast, but tornadoes have been known to move in any direction. The average speed is 30 mph, but some tornadoes remain nearly stationary while others travel at nearly 70 mph!

Tornado alley is an area in America where tornadoes are most common.

These areas can be seen highlighted in yellow.

This area is threatened by tornadoes as it is along a front line.

Economic Impacts

• Costs and losses to agricultural and livestock producers • Crop loss • Loss of livestock • Damage to houses, barns, and other farm buildings • Damage to farm machinery • Damage to fences

• Loss from timber production • Damage to and loss of trees • Urban, residential, and commercial impacts • Damage to and destruction of buildings • Loss of trees and landscaping • Damage to and destruction of automobiles, trucks, trains, airplanes, etc.

• Disruptions to local utilities and services • Power • Communications • Drinking water • Transportation

Social Impacts

• Health

• Injuries

• Fatalities

• Mental and physical stress associated with loss of family, friends, and property

Environmental Impacts• Kills animals

• Kills trees

• Destroys ecosystems

• Disrupts biodiversity

• Habitats lost

Case Studies

Pennsylvania Ohio Outbreak: 41 tornadoes during the late afternoon and evening of May 31, 1985, resulting in 75 deaths, 1,025 injuries, and $450 million in damage

 Plains Outbreak: 54 tornadoes from the afternoon of April 26 through early morning of April 27, 1991, resulting in 21 deaths, 308 injuries, and over $277 million in damage.

On July 18, 1996 the town of Oakfield, Wisconsin was all but destroyed by a powerful tornado. In addition to the tornado, strong thunderstorms brought heavy rains, lightning, and gusty winds to the region. All of these elements resulted from the passage of a cold front. The violent tornado developed in Fond du Lac County and moved southeastward across Wisconsin taking direct aim at Oakfield. At 7:15 PM the large tornado struck the 1,000 person town, injuring 17 people. Damage estimates totaled over $40 million as 47 of 320 homes were destroyed. In addition, 56 homes as well as numerous businesses and churches sustained heavy damage. A "state of emergency" was declared by Governor Tommy Thompson allowing National Guard soldiers to be called in to aid victims and clear debris.

Management

It is important to understand terms that you will hear during times of severe weather. The National Weather Service issues a tornado WATCH when conditions are favorable for the development of severe thunderstorms which could produce tornadoes. When a watch is issued for your area, continue with your activities but remain alert for signs of approaching storms (see Environmental Clue section), and continue to listen for weather forecasts and statements from your local National Weather Service office.

When a developing tornado has been detected by weather radar or one has been reported by trained storm spotters, your local National Weather Service office will issue a tornado WARNING. A tornado warning is an urgent message to people in the affected counties or parishes that a tornado is imminent. When a tornado warning is issued for your area, immediate action is required.

Tornadoes sometimes develop in areas where a SEVERE THUNDERSTORM watch or warning is in effect. Occasionally, tornadoes develop so rapidly that advance warning is not possible. You must remain alert for signs of approaching thunderstorms and be prepared to take shelter.

It is important to have a plan of action in the event a tornado threatens and to rehearse the plan frequently. Such a plan should include what you and your family should do when at home, work, school, or outdoors. The time spent planning NOW could determine whether or not you survive a tornado. Know the county or parish in which you live.

Severe weather warnings are issued for counties/parishes or for portions of a county/parish (i.e., northwest Jackson County or southern Vermilion Parish). By keeping a highway road map nearby, you can follow storm movements and better determine if you are threatened. Have a NOAA Weather Radio with a warning alarm tone to receive the latest information from your local National Weather Service office.

In a home or building, move to a predesignated shelter, such as a basement or the lowest floor of your home. Do not worry about which corner to go to in the basement. Instead, get under a sturdy piece of furniture or a workbench. If an underground shelter is not available, move to an interior room or hallway on the lowest floor and get under a sturdy piece of furniture

 Stay away from windows. Get out of automobiles. Tornadoes are unpredictable and can move faster than your car. If you are in a car and see a tornado, leave it immediately and get inside sturdy shelter.

 If caught outdoors and no sturdy shelter is available, lie flat in a nearby ditch or depression, and cover your head with your hands and arms. A high percentage of tornado fatalities occur due to objects striking the head.

The End