A tornado is a violently rotating column of air which descends from a thunderstorm to the ground. No other weather phenomenon can match the fury and destructive power of tornadoes. Tornadoes can be strong enough to destroy large buildings, leaving only the bare concrete foundation. In addition, they can lift 20-ton railroad cars from their tracks and they can drive straw and blades of grass into tree and telephone poles.
The truth is that scientists don't fully understand how tornadoes form. Typically, tornadoes develop several thousand feet above the earth's surface inside of a severe rotating thunderstorm. This type of storm is called a supercell thunderstorm. The spinning of these supercell thunderstorms is visible via Doppler radar.
A supercell is an organized thunderstorm that contains a very strong, rotating updraft. This rotation helps to produce severe weather events such as large hail, strong downbursts, and tornadoes. Supercell storms are usually isolated from other thunderstorms because it allows them to have more energy and moisture from miles around. These storms are rare, but always a threat to life and property.
A tornado begins as a rotating, funnel-shaped cloud extending from a thunderstorm cloud base. A funnel cloud is made visible by cloud droplets, however, in some cases it can appear to be invisible due to lack of moisture. When the funnel cloud is half-way between the cloud base and the ground, it is called a tornado. The tornado's high-speed winds rotate about a small, relatively calm center, and suck up dust and debris, making the tornado darker and more easily seen.
Tornado paths range from 100 yards to one mile wide and are rarely more than 15 miles long. They can last from several seconds to more than an hour, however, most don't exceed 10 minutes. Most tornadoes travel from the southwest to northeast with an average speed of 30 mph, but the speed has been observed to range from almost no motion to 70 mph.
Most tornadoes occur in the deep south and in the broad, relatively flat basin between the Rockies and the Appalachians, but no state is immune. Peak months of tornado activity in the U.S. are April, May, and June. However, tornadoes have occurred in every month and at all times of the day or night. A typical time of occurrence is on an unseasonably warm and sultry Spring afternoon between 3 p.m. and 9 p.m.
Tornadoes form under a certain set of weather conditions in which three very different types of air come together in a certain way. Near the ground lies a layer of warm and humid air, along with strong south winds. Colder air and strong west or southwest winds lie in the upper atmosphere. Temperature and moisture differences between the surface and the upper levels create what we call instability. A necessary ingredient for tornado formation. The change in wind speed and direction with height is known as wind shear. This wind shear is linked to the eventual development of rotation from which a tornado may form.
A third layer of hot dry air becomes established between the warm moist air at low levels and the cool dry air aloft. This hot layer acts as a cap and allows the warm air underneath to warm further...making the air even more unstable. Things start to happen when a storm system aloft moves east and begins to lift the various layers. Through this lifting process the cap is removed, thereby setting the stage for explosive thunderstorm development as strong updrafts develop. Complex interactions between the updraft and the surrounding winds may cause the updraft to begin rotating-and a tornado is born.
The Great Plains of the Central United States are uniquely suited to bring all of these ingredients together, and so have become known as "Tornado Alley." The main factors are the Rocky Mountains to the west, the Gulf of Mexico to the south, and a terrain that slopes downward from west to east.
During the spring and summer months southerly winds prevail across the plains. At the origin of those south winds lie the warm waters of the Gulf of Mexico, which provide plenty of warm, humid air needed to fuel severe thunderstorm development. Hot dry air forms over the higher elevations to the west, and becomes the cap as it spreads eastward over the moist Gulf air. Where the dry air and the Gulf air meet near the ground, a boundary known as a dry line forms to the west of Oklahoma. A storm system moving out of the southern Rockies may push the dry line eastward, with severe thunderstorms and tornadoes forming along the dry line or in the moist air just ahead of it.
Dr. T. Theodore Fujita, a pioneer in the study of tornadoes and severe thunderstorm phenomena, developed the Fujita Tornado Damage Scale (F-Scale) to provide estimates of tornado strength based on damage surveys. Since it is extremely difficult to make direct measurements of tornado winds, an estimate of the winds based on damage is the best way to classify them. The new Enhanced Fujita Scale (EF-Scale) addresses some of the limitations identified by meteorologists and engineers since the introduction of the Fujita Scale in 1971. Variability in the quality of construction and different local building codes made classifying tornadoes in a uniform manner difficult. In many cases, these inconsistencies led to overestimates in the strength of tornadoes. The new scale identifies 28 different free standing structures most affected by tornadoes taking into account construction quality and maintenance. The range of tornado intensities remains as before, zero to five, with 'EF0' being the weakest, associated with very little damage and 'EF5' representing complete destruction, which was the case in Greensburg, Kansas on May 4th, 2007, the first tornado classified as 'EF5'. The EF scale was adopted on February 1, 2007.
The Storm Prediction Center has a brief description of the new Enhanced Fujita Scale. Here's the full report submitted by the Wind Science and Engineering Center at Texas Tech University in PDF format.
A modification of the original Fujita Scale developed by "Dr. Tornado", T. Theodore Fujita of the University of Chicago.
|New EF Scale:||Old F-Scale:||Typical Damage:|
|EF0 (65-85 mph)||F0 (65-73 mph)||Light damage. Peels surface off some roofs; some damage to gutters or siding; branches broken off trees; shallow-rooted trees pushed over.|
|EF1 (86-110 mph)||F1 (73-112 mph)||Moderate damage. Roofs severely stripped; mobile homes overturned or badly damaged; loss of exterior doors; windows and other glass broken.|
|EF2(111-135 mph)||F2 (113-157 mph)||Considerable damage. Roofs torn off well-constructed houses; foundations of frame homes shifted; mobile homes completely destroyed; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground.|
|EF3 (136-165 mph)||F3 (158-206 mph)||Severe damage. Entire stories of well-constructed houses destroyed; severe damage to large buildings such as shopping malls; trains overturned; trees debarked; heavy cars lifted off the ground and thrown; structures with weak foundations blown away some distance.|
|EF4 (166-200 mph)||F4 (207-260 mph)||Devastating damage. Whole frame houses Well-constructed houses and whole frame houses completely leveled; cars thrown and small missiles generated.|
|EF5 (>200 mph)||F5 (261-318 mph)||Incredible damage. Strong frame houses leveled off foundations and swept away; automobile-sized missiles fly through the air in excess of 100 m (109 yd); high-rise buildings have significant structural deformation; incredible phenomena will occur.|
|EF No rating||F6-F12 (319 mph to speed of sound)||Inconceivable damage. Should a tornado with the maximum wind speed in excess of F5 occur, the extent and types of damage may not be conceived. A number of missiles such as iceboxes, water heaters, storage tanks, automoblies, etc.will create serious secondary damage on structures.|
Continued vigilance and quick response to tornado watches and warnings are critical, since tornadoes can strike virtually anywhere at any time. Most tornadoes are abrupt at onset, short-lived and often obscured by rain or darkness. The best way to deal with them is preparedness. Every individual and business should have a tornado emergency plan for their homes and places of work, and should learn how to protect themselves in cars, open country, and other situations that may arise.
Remember if a tornado warning is issued for your area, a tornado is imminent. Know what to do--have an emergency plan to protect yourself and those for whom you are responsible. Quick response when a tornado approaches can save many lives. There may be only seconds in which to take action.
There are 5 Basic Steps in the National Weather Service warning system. Every part of the system has to work for the greatest number of people to get the warning in time.
Meteorologists using the latest in computers, radar and satellite data are always monitoring the weather elements. When a high probability of severe weather exists, a tornado or severe thunderstorm watch is issued. Watches may be issued hours before any severe storm hits the area. The forecasters at the National Severe Storms Forecast Center in Kansas City, Missouri use every tool available including satellite pictures, radar reports, and numerous weather charts to predict the areas where severe thunderstorms and tornadoes are likely to occur.
Severe weather spotters are constantly being trained under the Skywarn training program. Spotting severe weatheris serious business and requires specific training. However, once an observer is trained in severe weather spotting procedures, they are one of the most reliable tools of the National Weather Service meteorologist. Spotters serve as the National Weather Service's eyes in the field.
Any information that the National Weather Service issues is relayed to individual cities and towns through state and local civil defense, and the State Police.
The vast majority of people are reached through the cooperation of the media. A direct line between the National Weather Service and local media offices insures that severe weather information is relayed quickly and broadcast within the shortest time possible.
Users include everyone within the severe thunderstorm or tornado warning area. We want to reach the greatest number of people possible and provide a concise, yet persuasive message of necessary action. Even if every other step in the warning system works, it does little good unless the users know what to do, and act.
The deadliest tornado outbreak in U.S. history occured on March 18, 1925. 747 people were killed and 2,027 were injured in Missouri, Illinois, and Indiana when several twisters touched down on this day. The largest of these tornadoes, named the "Tri-State", took 695 lives and was classified as an F5. It moved over 215 miles of land at 60-73 mph.
The longest tornado path was the Tri-State tornado of March 18, 1925, with a path length of 215 miles. It was on the ground for 7 hours and 20 minutes. Because there was no damage report filed, its difficult to know if this tornado actually stayed on the ground for the entire time, or if it was the result of several tornadoes. The shortest reported tornado was 7 feet long.
Mobile Doppler radar reported one twister on May 3, 1999 as having winds as fast as 302 mph, plus or minus 16 mph, at an altitude of 100 feet. Scientists have found that the strongest winds typically occur about 300 feet above the ground. Most tornadoes, however, fail to have wind speeds in excess of 113 mph.
NO! Opening your windows and doors may in fact increase the damage to your house and make you suseptible to being struck by flying glass. Instead, use that time to find a safe spot under heavy furniture and away from windows. Any openings, including garage doors, that allow wind to enter a building increases the chance for damage.
The number of tornadoes that occur each year is not increasing, but the number of spotted and reported tornadoes is. The reason for this is that more people live in or travel through tornado prone areas than used to. This has led to better communication and reportings of severe weather.
In addition to the thousands of National Weather Service trained severe weather spotters, NEXRAD Doppler radars detect severe weather. These radars spot large scale rotation from which many tornadoes form. NEXRAD does not detect every tornado, but it is likely that they will provide advance warning for large twisters.
We can do little to prevent a tornado from occurring, but by knowing the safety rules we can minimize the number of deaths and injuries.
A tornado watch means that tornado development is possible. Keep a watchful eye on the sky for threatening weather and stay tuned to radio and television and listen for weather bulletins.
A tornado warning means that a tornado has been sighted or indicated by radar. Persons in the path of the storm should seek shelter immediately - preferably in a storm cellar, underground excavation, or in a steel-framed or concrete reinforced building.
A severe thunderstorm warning means that either spotters or radar have indicated that severe weather is occuring, and is expected to be heading towards you soon. This warning is issued by the National Weather Service local office, and usually covers a few counties, lasting about an hour or so. A thunderstorm is classified as severe because it can contain hail three-quarter inches or larger, and/or wind gusts of 58 mph or higher, and/or a tornado. When a warning is issued, persons should remain indoors until the storm has passed.
In homes,the basement offers the greatest safety. Seek shelter under sturdy furniture if possible. In homes without basements take cover in the center part of the house, on the lowest floor, in a small room such as a closet or bathroom, or under sturdy furniture. Stay away from windows!
In schools, hospitals, and shopping centers move to pre-designated shelter areas. Interior hallways on lowest floors are best. If the building is not of reinforced construction, go to a nearby one that is, or take cover outside on low, protected ground. Stay out of auditoriums, gymnasiums, and other structures with wide free-span roofs.
In open country, move away from the tornado's path at right angles. If there is not time to escape,lie flat in the nearest ditch or ravine.
In your car, do not try to outrun a tornado. If available take shelter in a sturdy structure. Otherwise, get in the nearest ditch or depression until the tornado passes.
Mobile homes are particularly vulnerable to overturning during strong winds and should be evacuated when strong winds or tornadoes are forecast. Damage can be minimized by securing trailers with cables anchored in concrete footing. Trailer parks should have some community storm shelters. If there is no shelter nearby, leave the trailer park and take cover on low-protected ground.