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 mph 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 tornados?

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 verywarm, 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.

 

Tornado Variations

  • 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.

Weak Tornadoes

  • 69% of all tornadoes
  • Less than 5% of tornado deaths
  • Lifetime is 1-10+ minutes
  • Winds less than 10 mph

Strong Tornadoes

  • 29% of all tornadoes
  • Nearly 30% of all tornado deaths
  • May last 20 minutes or longer
  • Winds 110-205 mph

Violent Tornadoes

  • Only 2% of all tornadoes
  • 70% of all tornado deaths
  • Lifetime can exceed 1 hour
  • Winds over 205

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.
  • Note, in some states, a secondary tornado maximum occurs in the fall.
  • Tornadoes are most likely to occur between 3 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 nearly stationary to 70 mph.
  • The total number of tornadoes is probably higher than indicated in the western states. Sparse population reduces the number reported.

Environmental Clues

Look out for:

  • Dark, often greenish sky
  • Wall cloud
  • Large hail
  • Loud roar; similar to a freight train

Fujita Tornado Scale


WINDS DAMAGE DAMAGE DETAIL
F0 Up to 72 mph Light damage chimneys damaged, tree branches broken, shallow-rooted trees toppled.
F1 73-112 mph Moderate damage roof surfaces peeled off, windows broken, some tree trunks snapped, unanchored mobile homes overturned, attached garages may be destroyed.
F2 113-157 mph Considerable damage roof structures damaged, mobile homes destroyed, debris becomes airborne, large trees snapped or uprooted.
F3 158-206 mph Severe damage roofs and some walls are torn from structures, small buildings destroyed, non-reinforced masonry buildings destroyed, most trees in forest are uprooted.
F4 207-260 mph Devastating well-constructed houses destroyed, some structures lifted from foundations and blown some distance, cars are blown some distance, large debris becomes airborne.
F5 Above 261 mph Incredible strong frame houses lifted from foundations, reinforced concrete structures damaged, automobile-sized missiles become airborne, trees completely debarked.