Although tornadoes occur in many parts of the world, these destructive forces of nature are found most frequently in the United States east of the Rocky Mountains during the spring and summer months. In an average year, 800 tornadoes are reported nation-wide, resulting in 80 deaths and over 1,500 injuries. A tornado is defined as 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 miles per hour or more. Damage paths can be in excess of 1 mile wide and 50 miles long.
See the National Weather Service's Storm Prediction Center for information about the Enhanced F Scale for Tornado Damage.
What Causes Tornadoes?
Thunderstorms develop in warm, moist air in advance of eastward-moving cold fronts. These thunderstorms often produce large hail, strong winds, dangerous lightning, and tornadoes. During the spring in the Central Plains, thunderstorms frequently develop along a "dry" line, which separates very warm, moist air to the east from hot, dry air to the west. Tornado-producing thunderstorms may form as the dry line moves east during the afternoon hours.
Frequency of Tornadoes
- Tornadoes can occur at any time of the year.
- In the southern states, peak tornado occurrence is in March through May, while peak months in the northern states are during the summer.
- Tornadoes are most likely to occur between 3 p.m. and 9 p.m., but have been known to occur at all hours of the day or night.
- The average tornado moves from southwest to northeast, but tornadoes have been known to move in any direction. The average forward speed is 30 mph, but may vary from stationary to 70 miles per hour.
- Dark, often greenish sky
- Wall cloud
- Large hail
- Loud roar similar to a freight train
- Some tornadoes appear as a visible funnel extending only partially to the ground. Look for signs of debris below the visible funnel
- Some tornadoes are clearly visible while others are obscured by rain or nearby low-hanging clouds
Other Thunderstorm Hazards
- Flash floods are the number 1 weather killer with over 146 deaths annually
- Lightning kills 60 - 100 people per year
- Damaging straight-line winds can reach over miles per hour
- Large hail which can reach the size of a grapefruit
Myths About Tornadoes
- Tornadoes are always visible from a great distance.
- False - They can be hidden in heavy rainfall
- Tornadoes cause houses to explode from changes in air pressure.
- False - Homes are damaged by strong winds and debris, not air pressure changes
- By opening windows, you can balance the pressure inside and outside your home so a tornado will not do damage.
- False - The force of a tornado can rip through a structure whether the windows are open or not.
- The best place to be during a tornado is generally in the southwest corner of a room or basement.
- False - This used to be a safety rule based on the idea that debris would usually not be deposited there, but this has now been rethought. The current best advise is to move to a protected interior room on the lowest floor of a building, as far away as possible from exterior walls and windows
- Tornadoes cannot cross water.
- False - A waterspout is a type of tornado that forms on water, and tornadoes that form on land can cross bodies of water such as rivers and lakes. Tornadoes, especially the more violent ones, can also travel up and down hillsides
- A tornado is always accompanied or preceded by a funnel cloud.
- False - Especially in the early stages, a tornado can cause damage on the ground even though a visible funnel cloud is not present. Likewise, if you see a funnel cloud but it does not appear to be "touching down", a tornadic circulation nonetheless may be in contact with the ground
- Downward-bulging clouds mean tornadoes are on the way.
- Not necessarily - This may be the case, especially with those that show evidence of a rotating motion, but many of these clouds are not associated with tornadoes and may be completely harmless
A tornado watch is issued by the National Weather Service when weather conditions which may produce tornadoes are expected to develop.
Continue normal activities, but listen to NOAA Weather Radio or local radio/TV. Stay alert for abrupt changes in weather conditions such as darkening skies, hail, or increased wind.
A tornado warning is issued by the National Weather Service when Doppler radar indicates tornado formation or a tornado has been sighted by a trained weather spotter.
The warning will tell the tornado location, and if possible, movement, estimated speed, and cities in the tornado path.
Persons close to the tornado location should take cover in a basement or lowest floor interior room immediately. A few seconds could save your life.
Other Safety Measures if You See a Tornado
- Seek inside shelter immediately, a basement or underground shelter is best. Protect your head and eyes from flying debris. Take a portable radio or TV with you to the shelter.
- If no basement is available, go to the interior part of the lowest floor, such as a bathroom or closet. Stay away from windows. Get under something sturdy like a bench or table.
- Mobile Homes are particularly vulnerable to overturning and destruction during strong winds and tornadoes. Tie downs will generally not protect a mobile home during a tornado. If possible, leave the mobile home and go to a neighbor's house or community shelter. If none is available, a ditch or culvert may offer better protection. Prepare a plan of action before the storm hits.
- In schools, follow advance plans to a basement, an interior hallway, or the lowest floor. Avoid the ends of hallways that open to the outside. Avoid classrooms with windows or outside walls. Stay out of auditoriums, gymnasiums, or other structures with wide, free-span roofs. Rest rooms in the middle of the building can offer some shelter from flying debris.
- Do not board or stay on a school bus during a tornado warning. School buildings offer more protection.
- If you are in a vehicle, abandon the car or truck and seek refuge in a basement, storm shelter, or sturdy building. As a last resort, seek shelter in a culvert or ditch. Protect your head and body from flying debris.
- If you are in an office building, hotel, or shopping mall, go to the basement, designated shelter, or the center of the building on the lowest level, as upper stories can be unsafe. Stay away from large, open rooms and windows. Never seek shelter in cars in the parking lot. Buildings with large, free-span roofs are very vulnerable to tornadoes. Occupants should leave these areas and move to smaller interior rooms, basements, or designated storm shelters.
- If you are in open country, seek shelter if shelter is available and time permits. If there is no time or shelter, lie flat in the nearest depression, such as a ditch or culvert, and protect your head with your arms.