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 nationwide, 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 one mile wide and 50 miles long. Once a tornado in Broken Bow, Oklahoma, carried a motel sign 30 miles and dropped it in Arkansas!
What Causes Tornadoes?
Thunderstorms develop in warm, moist air in advance of eastward-moving cold fronts. These thunderstorms often produce large hail, strong winds, and tornadoes. Tornadoes in the winter and early spring are often associated with strong, frontal systems that form in the Central States and move east. Occasionally, large outbreaks of tornadoes occur with this type of weather pattern. Several states may be affected by numerous severe thunderstorms and tornadoes.
During the spring in the Central Plains, thunderstorms frequently develop along a "dryline," which separates very warm moist air to the east from hot, dry air to the west. Tornado-producing thunderstorms may form as the dryline moves east during the afternoon hours.
Along the Front Range of the Rocky Mountains, in the Texas panhandle, and in the southern High Plains, thunderstorms frequently form as air near the ground flows "upslope" toward higher terrain. If other favorable conditions exist, these thunderstorms can produce tornadoes.
Tornadoes occasionally accompany tropical storms and hurricanes that move over land. Tornadoes are most common to the right and ahead of the path of the storm center as it comes onshore.
- Some tornadoes may form during the early stages of rapidly developing thunderstorms. This type of tornado is most common along the Front Range of the Rocky Mountains, the Plains, and the Western States.
- Tornadoes may appear nearly transparent until dust and debris are picked up.
- Occasionally, two or more tornadoes may occur at the same time.
- Waterspouts are weak tornadoes that form over warm water.
- Waterspouts are most common along the Gulf Coast and southeastern states. In the western United States, they occur with cold late fall or late winter storms, during a time when you least expect tornado development.
- Waterspouts occasionally move inland becoming tornadoes causing damage and injuries.
How Do Tornadoes Form?
Before thunderstorms develop, a change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere.
Rising air within the thunderstorm updraft tilts the rotating air from horizontal to an area of rotation, two to six miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.
Tornado Shapes and Sizes
69percent of all tornadoes
Less than five percent of tornado
Lifetime one to 10 or more minutes
Winds less than 110 mph
29 percent of all tornadoes
Nearly 30 percent of all tornado deaths
May last 20 minutes or longer
Winds 110 to 205 mph
Only two percent of all tornadoes
70 percent of all tornado deaths
Lifetime can exceed one hour
Winds greater than 205 mph
Myth: Areas near rivers, lakes, and mountains are safe from tornadoes.
Fact: No place is safe from tornadoes. In the late 1980's, a tornado swept through Yellowstone National Park leaving a path of destruction up and down a 10,000 foot mountain.
Myth: The low pressure with a tornado causes buildings to "explode" as the tornado passes overhead.
Fact: Violent winds and debris slamming into buildings cause most structural damage.
Myth: Windows should be opened before a tornado approaches to equalize pressure and minimize damage.
Fact: Opening windows allows damaging winds to enter the structure. Leave the windows alone; instead, immediately go to a safe place.
Frequency of Tornadoes
Tornadoes can occur at any time of the year. In our area, they are most common beginning in May and continuing through the summer months. They usually are associated with strong thunderstorm activity and strong fronts.
Tornadoes occur at any hour 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 miles per hour but may vary from nearly stationary to 70 miles per hour.
Stay Informed About the Storm
By listening to NOAA Weather Radio, commercial radio, and television for the latest tornado watches and warnings.
When conditions are favorable for severe weather to develop, a severe thunderstorm or tornado watch is issued.
Weather Service personnel use information from weather radar, spotters, and other sources to issue severe thunderstorm and tornado warnings for areas where severe weather is imminent.
Severe thunderstorm warnings are passed to local radio and television stations and are broadcast over local NOAA Weather Radio stations serving the warned areas. These warnings are also relayed to local emergency management and public safety officials who can activate local warning systems to alert communities.
NOAA Weather Radio is the best means to receive warnings from the National Weather Service
The National Weather Service continuously broadcasts updated weather warnings and forecasts that can be received by NOAA Weather Radios sold in many stores. The average range is 40 miles, depending on topography. Your National Weather Service recommends purchasing a radio that has both a battery backup and a tone-alert feature that automatically alerts you when a watch or warning is issued.
Tornado Watch: Tornadoes are possible in your area. Remain alert for approaching storms.
Tornado Warning: A tornado has been sighted or indicated by weather radar. If a tornado warning is issued for your area and the sky becomes threatening, move to your pre-designated place of safety.
Severe Thunderstorm Watch: Severe thunderstorms are possible in your area.
Severe Thunderstorm Warning: Severe thunderstorms are occurring. Remember that tornadoes occasionally develop in areas in which a severe thunderstorm watches or warning is in effect. Remain alert to signs of an approaching tornado and seek shelter if threatening conditions exist.
Look out for:
- Dark, often greenish sky
- Wall cloud
- Large hail
- Loud roar; similar to a freight train
Some tornadoes appear as visible funnels extending 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.
Revised June 5, 2014