The National Weather Service defines a tornado as a violently rotating column of air pendant from a thunderstorm cloud and touching the ground
From a local perspective, a tornado is the most destructive of all atmospheric-generated events. In an average year, over 600 tornadoes strike various parts of the United States. About half of them occur in the period April through June.
TIME OF DAY during which tornadoes are most likely to occur is mid-afternoon, from
3 to 7 p.m.
DIRECTION OF MOVEMENT is predominantly from the southwest to the northeast. Tornadoes associated with hurricanes may move from an easterly direction. About 85 percent of all tornadoes come from the southwest, plus or minus 45 degrees. Directions may vary significantly in local areas.
LENGTH OF PATH averages 4 miles, but some have exceeded 100 miles.
WIDTH OF PATH averages 300-400 yards, but may reach up to 1 mile.
SPEED OF TRAVEL (TRANSLATIONAL) averages from 25 to 40 m.p.h., but speeds from 0 to 70 m.p.h. have been recorded.
SPEED OF TRAVEL (ROTATIONAL) is assumed to be symmetrical. The maximum rotational velocity occurs at the edge of the tornado core. Speed reduces rapidly as the distance from the edge increases.
INTENSITY OF DAMAGE from a tornado is related to windspeed (assuming consistent building construction).
The F rating scale was developed by Dr. T, Throdore Fujita. It is based upon trained meteorologists' evaluation of damage following tornadoes.
Eighty-four percent of recent tornadoes have been rated at FO - 20% Light, F1 - 43% Moderate, F2 - 25% Considerable
Building damage is caused by a buildup of pressure on building surfaces, caused by the effects of high winds. This pressure is related to wind velocity squared
Overpasses Not Adequate
Tornado Protection
by Jim Packett, Meteorologist
NWS, Paducah, KY
On The Horizon newsletter reprinted with permission
Ever since the 1991 nationally televised clip of a TV news crew and several others successfully seeking safe shelter under an Andover, Kansas interstate overpass during a tornado, the general public has had the perception that overpasses are a good source of protection from tornadoes. This may be true in some instances. Yet, in the May 3, 1999 tornado outbreak in central Oklahoma, many people received injuries and several perished as a result of seeking shelter under an overpass.
The tornadoes in Oklahoma City were of a violent nature, sometimes measuring F5 on the tornado intensity scale. With tornadoes of this intensity, the only true place of safety is in an underground storm shelter. F5 tornadoes tend to level everything in their path.
One of the problems with an under-the-girder-type of shelter is that it can cause a dangerous wind tunnel effect. This may cause the winds to be stronger and more focused underneath. This can also cause the overpass to be a collector of debris.
Remember...do not try to outrun a tornado in your car. Instead leave it immediately. Seek shelter in a nearby sturdy building or underground shelter. If none is available, lie flat in the nearest low spot, dip or ditch.
Could A Tri-State Tornado Happen Again?
By Jim Kramper
National Weather Service
Warning Coordination Meteorologist
St. Louis, MO
Missouri Severe Weather Awareness Week 2000 will take place from March 13 - 17. The 26th annual Statewide Severe Weather Drill will be on Tuesday, March 14, around 1:30 p.m. The drill will take place as scheduled unless there is a serious threat of severe weather. The back-up day will be Thursday, March 16, also around 1:30 p.m.
March 18, 2000, will be the 75th anniversary of what is believed to be the single worst tornado event in United States history; The Tri-State Tornado. The tornado (or family of tornadoes, no one is really sure) started near Ellington, MO, in Reynolds County, at about 1:00 p.m. The tornado moved northeast for 219 miles across southeast Missouri, southern Illinois and finally into southwest Indiana before dissipating. For three and a half hours, more people would die, more schools would be destroyed, more students and farmers would be killed, and more deaths would occur in a single city than from any tornado in U.S. history. Records were also set for speed, path length, and probably for other categories that can’t be measured so far in the past. Close to 700 people died and a little over 2000 were injured. The majority of the deaths and injuries occurred in Illinois and Indiana. Most records indicate there were 11 deaths in Missouri.
Can a Tri-State tornado happen again? A tornado that powerful is certainly possible. One needs only to remember the Oklahoma City tornado of this year to realize that extremely powerful tornadoes can happen any year. One thing that will be different though will likely be the death toll. A lot has happened since 1925. For one thing, a tornado of this magnitude will not be a surprise. By utilizing the WSR-88D radar and our trained network of Severe Storm Spotters, the National Weather Service (NWS) will know about the tornado and will have warnings out. The media will broadcast the warnings enabling thousands of people to know what is coming. This didn’t happen in 1925.
In many areas, homes are built better than they were in 1925. Basements are more common now providing shelter that may not have been available in years past. And today, in homes without basements, building a "shelter room" in the home is becoming increasingly popular.
Yes, we are in better shape today. But we cannot rest and expect everything to be OK. We must continue to make advancements in detection and mitigation. So ask yourself, is your community ready if a deadly tornado heads your way? Are there storm spotters helping the NWS locate the storm? Can the community receive the warnings in a timely manner, either via NOAA Weather Radio or the commercial media. Will schools, hospitals and other public facilities be notified? No one wants another Tri-State, but if it happens, hopefully your community will be as ready as possible.
Effects of High Winds (.pdf
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