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HomeMy WebLinkAboutTornado Facts 1976 1 TORNADO FACTS Of all the winds that sweep the earth's surface, tornadoes are by far the most terrible. Their time is short and their paths of destruction generally rather small, but their march through populated areas leaves scenes of awful devastation. A tornado funnel can reduce solid buildings to matchwood, convert a straw into a deadly missle, uprooi huge trees, and ..arl people and animal for hundreds of yards. When there is such complete destruction there is usually also loss of life. On April 11, 1965, Palm Sunday, 37 tornadoes struck the midwest, killing more than 250 persons and injuring more than 5000; property damage was estimated at $300 million. Since the early 1950's, the tornado death toll has averaged 122 per year, and the damage $40 million annually, although it is often much higher. Tornadoes occur in all 50 states. But no area is more susceptible to their formation than the continental plains of North America, and no season is free of them. Normally, they are least frequent in the United States during December and January, and at the peak in May. The months of greatest frequence are April, May and June. -2- Tornadoes may occur at any hour of the day or night, but, because of the meteorological combinations which create them, they form most readily during the warmest hours of the day. The greatest number -- 82 per- cent of the total -- occurs between noon and midnight, and the greatest single concentration -- 23 percent of total tornado activity -- falls between 4 and 6 p.m. During the period 1953 -1967, an average of 643 tornadoes per year occurred in the United States, about half of them during three months -- April, May, and June. For the same period, the annual average number of tornado days -- days on which one or more tornadoes were reported -- was 157. Average annual frequency by states for this period ranges from 115 tornadoes in Texas to less than three in most of the northeastern and far western states. On the average, tornado paths are only a quarter of a mile wide and seldom more than 16 miles long. But there have been spectacular instances in which tornadoes have caused heavy destruction along paths more than a mile wide and 300 miles long. A tornado traveled 293 miles across Illinois, and Indiana on May 26, 1917, and lasted 7 hours and 20 minutes. Its forward speed was 40 miles an hour, an average figure for tornadoes. Tornadoes are formed of winds rotating at very high speeds, usually counterclockwise. These storms are visible as a vortex, a whirlpool -3- structure of winds rotating about a hollow cavity in which centrifugal forces produce a partial vaccum. As condensation occurs around the vortex, a pale cloud appears -- the familiar and frightening tornado funnel. Air surrounding the funnel is also part of the tornado vortex; as the storm moves along the ground, this outer ring of rotating winds becomes siirk with dust and debris, which may eventually darken the entire funnel. Tornadoes do their destructive work through the combined action of their strong rotary winds and the partial vacuum in the center of the vortex. As a tornado passes over a building, the winds twist and rip as the outside at the same time that the abrupt pressure reduction in the tornado's "eye" causes explosive over - pressures inside the building. Walls collapse or topple outward, windows explode, and debris of this destruction is driven through the air in a dangerous barrage. PROTECTION AGAINST TORNADOES The Weather Bureau, a major element of the Commerce Department's • Environmental Science Services Administration, provides the Nation's first line of defense against destructive natural hazards. Through its _4_ tornado watches and warnings, the Weather Bureau gives those in threatened areas time to seek shelter. It is important to know the difference between a watch and a warning, and to know how to react to each. The Watch Tornado watches are the first alerting message between the Weather Bureau's National Servere Storms Forecast Center and areas potentially threatened by tornadoes. They specify the area covered by the watch, and establish a period of time during which tornado probabilities are expected to be dangerously high. Watches are teletyped directly to local office of the Weather Bureau and disseminated by those offices to the public via radio and television stations in and around endangered areas. Law enforcement officers, emergency forces, volunteer storm reporters and other cooperating personnel are also alerted by the watches, and they relay the alert to others in the watch area. Watches are not warnings. They are issued to alert persons to the possibility of tornado development in a specified area, for a specified period of time. Until a tornado warning is issued, persons in watch areas should not interrupt their normal routines except to watch for threatening weather. -5- The Warning Tornado warnings are issued when a tornado has actually been sighted in the area or indicated by radar. In many cases, they are made possible through the cooperation of public- spirited persons who notify the nearest Weather Bureau office or community warning center when a tornado is sighted. Warnin:s indicate the location of the tornado at the time of detection the area throu which it is expected to move and the time period during which the tornado will move through the area warned. When a tornado warning is issued, persons in the path of the storm should take immediate safety precautions. Much of the burden of warning, evacuation, and shelter falls to communities and individuals. Tornado detection requires a dense network of storm reporters and a reporting procedure within each tornado watch area. The Weather Bureau receives help from nearly 500 local networks, and could use the help of many more. Network observers and the public are alerted to the possibility of dangerous weather by the tornado watches issued by the Weather Bureau. When a watch is in effect, observers are alerted to notify the nearest Weather Bureau office as soon as a tornado is sighted, describing the type of storm, its location, intensity, and direction of movement. -6- SAFETY RULES Nobody who has lived through a tornado will ever underestimate its terrible potential for dealing death and destruction. Thousands who have never seen a tornado, or viewed the way in which it can transform a happy neighborhood into a mass of rubble may tend to theory that "it can't happen here." Statistics show it can it has. The ESSA Weather Bureau em- phasizes, in the strongest possible terms, that familiarity with the following safety rules may well mean the difference between life and death. When you receive a tornado warning, seek inside shelter, preferably in a tornado cellar, underground excavation, or steelframed or reinforced concrete building. - -- Stay away from windows. - -- If you are in an office building, stand in an interior hallway on a lower floor, or basement. - -- Factory workers should post a look -out and move to the section of the plant offering greatest protection - -- In homes without tornado shelters, the basement usually offers greatest safety. - -- Seek shelter under heavy furniture in the center of house if you have no basement. • -7- - -- Keep some windows oper, but stay away from them. - -- Do not stay in mobile homes when a tornado warning is received. - -- In schools, go to an interior hallway or basement shelter; avoid auditoriums, gymnasiums, and other structures with wide, free - span roofs. - -- If you see a tornado -- and if there is time -- report it to your local Weather Bureau office, or to law enforcement agencies. - -- During tornado emergencies, stay tuned to radio or television for lstest messages from the Weather Bureau.