Tornadoes: The What, When & Where

Tornadoes are Nature’s Most Violent Storms

People, recreational vehicles, cars, and even buildings may be hurled aloft by tornado-force winds—or simply blown away. Most injuries and deaths are caused by flying debris.

A tornado is a vertical funnel of violently rotating air extending from a thunderstorm to the ground.

The most violent tornadoes are capable of tremendous destruction with wind speeds of 250 miles (400 kilometers) per hour or more and can clear-cut a pathway in excess of one mile (1.6 kilometers) wide and 50 miles (80 kilometers) long.

Once a tornado in Broken Bow, Oklahoma, carried a motel sign 30 miles and dropped it in Arkansas!

These violent storms occur in many parts of the world, but the United States is the major hotspot with over 800 tornadoes reported every year. “Tornado Alley,” a region that includes eastern South Dakota, Nebraska, Kansas, Oklahoma, northern Texas, and eastern Colorado, is home to the most powerful and destructive of these storms. U.S. tornadoes cause 80 deaths and more than 1,500 injuries per year.

Canada gets more tornadoes than any other country with the exception of the United States. Tornadoes are relatively common in Canada, but only in specific regions: southern portions of Alberta, Saskatchewan, Manitoba, Ontario, and Quebec. Tornado season in Canada extends from April to September with peak months in June and July, but they can occur at any time.

Tornadoes’ distinctive funnel clouds are actually transparent. They become visible when water droplets pulled from a storm’s moist air condense or when dust and debris are taken up. Funnels typically grow about 660 feet (200 meters) wide.

Tornadoes move at speeds of about 10 to 20 miles (16 to 32 kilometers) per hour, although they’ve been clocked in bursts up to 70 miles (113 kilometers) per hour. Most don’t get very far though. They rarely travel more than about six miles (ten kilometers) in their short lifetimes.

Tornado forecasters can’t provide the same kind of warning that hurricane watchers can, but they can do enough to save lives. Today the average warning time for a tornado alert is 13 minutes.

Tornadoes can also be identified by warning signs that include a dark, greenish sky, large hail, and a powerful train-like roar.

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.

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.

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.

Frequency of Tornadoes

The meteorological factors that drive tornadoes make them more likely at some times than at others. They occur more often in the late afternoon, when thunderstorms are common and are more prevalent in spring and summer. However, tornadoes can and do form at any time of the day and year.

In the southern states, peak tornado occurrence is in March through May, while peak months in the northern states are during the summer.