1. About Tornadoes
  2. Myths
  3. Tornado Safety

About Tornadoes

Tornado Information

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.

Environmental Clues

  • 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