Tornadoes

Characteristics


A tornado is a narrow, violently rotating column of air that extends from the base of a thunderstorm to the ground. It is hard to see a tornado until it forms condensation within the funnel made of water droplets, dust or debris.

Tornadoes are the most violent of all atmospheric storms. While tornadoes occur throughout the world, the United States averages around 1,200 a year, one of the highest concentrations in the world.

Storm Development


Tornadoes form primarily from supercell thunderstorms, those with well defined radar circulation. A rotating updraft is key to the development of the supercell and eventually the tornado itself. Often winds at ground level are slowed by friction while winds in the upper levels of the supercell move much faster. 

Rising air, often warm and moist, begins to develop the "tube" which then creates vertical rotation. This vertical updraft fed by the warm moist air can lead to the formation of the tornado. 

Tornadoes can also form in non-supercell thunderstorms, hurricanes and other coastal storms. The National Weather Service reports that on average only 20% of supercell storms will develop tornadic activity. 

Damages & Danger


Damages caused by tornadoes are usually the result of the high wind velocities and wind-blown debris. Lightening and hail usually accompany these storms and can also result in damages.

On average, tornadoes kill about 60 people per year, mostly from falling or flying debr

Occurrence


Peak tornado season in the Southeast is March through May and in the northern states it runs late spring through early summer. They can occur at any time of the day, but are most likely to occur between 3 and 9 p.m. Structures of light construction, such as mobile homes, are at greatest risk for damages from tornadoes.

Expand the map to view torando occurrence in the United States.

Map showing the yearly average tornado occurrence in the United States
According to the National Oceanic and Atmospheric Administration, the Storm Predication Center in Oklahoma City has the highest concentration of tornado activity for a city. North Carolina and Onslow County have approximately 1-6 tornadoes per average year.

The states with the highest concentration of tornados are:
  • Florida
  • Illinois
  • Iowa
  • Kansas
  • Oklahoma

Magnitude Scale


A tornado is reported by its magnitude according to the Enhanced Fujita Scale (EF Scale). The EF Scale became operational on February 1, 2007 replacing the original Fujita Scale. The scale assigns a "rating" based on estimated wind speeds and related damage.

After a tornado occurs, field surveyors will compare a list of Damage Indicators (DI) and degrees of damage to help estimate the range of wind speeds that were likely produced. The highest radar motion detected was over 300 mph.
EF Rating 3 Second Gust  Expected Types of Damages
EF0 65-85 mph Some damage to chimneys; breaks branches off trees; pushes over shallow-rooted  trees; damages to sign boards
EF1 86-110 mph The lower limit is the beginning of hurricane wind speed; peels surface off roofs; mobile  homes pushed off foundations or overturned; moving autos pushed off the roads; attached garages may be destroyed.
EF2 111-135 mph Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars  pushed over; large trees snapped or uprooted; light object missiles generated.
EF3 136-165 mph Roof and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted.
EF4 166-200 mph Well-constructed houses leveled; structures with weak foundations blown off some  distance; cars thrown and large missiles generated.
EF5 200 mph + Strong frame houses lifted off foundations and carried considerable distances to  disintegrate; automobile sized missiles fly through the air in excess of 100 meters; trees debarked; steel re-enforced concrete structures badly damaged. 

Waterspouts


Waterspouts are weak tornadoes that form over water and are most common along the Gulf Coats and Southeastern United States. Waterspouts will occasionally move inland, becoming a tornado that can cause damage and injury.

Most waterspouts will dissipate over open water causing the highest threat to marine and boating interests. Typically, waterspouts are weak and short lived and most go unreported unless they cause damages.

Tornado Safety

  1. Before a Tornado
  2. During a Tornado
  3. After a Tornado

What to do before a tornado



  • Listen to NOAA Weather Radio or radio or television newscasts for the latest updates. In any emergency, always listen to the orders given by local emergency management officials.
  • Be alert to changing weather. Look for oncoming storms.
  • Look for the following danger signs:
    • Dark, often greenish sky.
    • Large hail.
    • A large, dark, low-lying cloud (particularly if rotating).
    • Loud roar, much like a freight train.
    • If you see oncoming storms or any of these danger signs, be ready to take shelter quickly.
  • Know where to go. The safest place to be during a tornado is in a basement. If you have no basement, go to an inner hallway or smaller inner room without windows, such as a bathroom or closet. Go to the center of the room. Try to find something sturdy you can get under and hold onto to shield you from flying debris and/or a collapsed roof. Use your arms to protect your head and neck.
  • Mobile homes, even those with tie-downs, are particularly open to damage from high winds. Go to a prearranged shelter when the weather turns bad.
  • If no shelter is available, go outside and lie on the ground, if possible in a ditch or depression. Use your arms to protect your head and neck and wait for the storm to pass. While waiting, be alert for the flash floods that sometimes come with tornadoes.
  • Never try to outrun a tornado in a car. A tornado can toss cars and trucks around like toys. If you see a funnel cloud or hear a tornado warning issued, get out of your vehicle and find safe shelter. If no shelter is available, lie down in a low area using your arms to cover the back of your head and neck. Be sure to stay alert for flooding.

Know the Terms


Familiarize yourself with these terms to help identify a tornado hazard:

Tornado Watch - Tornadoes are possible. Remain alert for approaching storms. Watch the sky and stay tuned to NOAA Weather Radio, commercial radio or television for information.

Tornado Warning - A tornado has been sighted or indicated by weather radar. Take shelter immediately.

Additional Resources



Want to learn more about tornadoes and how to be prepared? Visit these site for more inofrmation: