15.7 Tornadoes

Who killed the Wicked Witch of the East?

Dorothy's house flies up in a tornado to the magical land of Oz. When the tornado ends, the house it falls on the witch. Dorothy becomes a hero for killing the tyrannical witch, but despite that yearns for home. In the real world, tornadoes do kill, but houses don't usually fly, and wicked witches usually avoid tornadoes.

Tornadoes

Tornadoes, also called twisters, are fierce products of severe thunderstorms (Figure below). As air in a thunderstorm rises, the surrounding air races in to fill the gap. This forms a tornado, a funnel-shaped, whirling column of air extending downward from a cumulonimbus cloud.

A tornado lasts from a few seconds to several hours. The average wind speed is about 177 kph (110 mph), but some winds are much faster. A tornado travels over the ground at about 45 km per hour (28 miles per hour) and goes about 25 km (16 miles) before losing energy and disappearing (Figure below).

Damage

An individual tornado strikes a small area, but it can destroy everything in its path. Most injuries and deaths from tornadoes are caused by flying debris (Figure below). In the United States an average of 90 people are killed by tornadoes each year. The most violent two percent of tornadoes account for 70% of the deaths by tornadoes.

Location

Tornadoes form at the front of severe thunderstorms. Lines of these thunderstorms form in the spring where where maritime tropical (mT) and continental polar (cP) air masses meet. Although there is an average of 770 tornadoes annually, the number of tornadoes each year varies greatly (Figure below).

April 2011

In late April 2011, severe thunderstorms pictured in the satellite image spawned the deadliest set of tornadoes in more than 25 years. In addition to the meeting of cP and mT mentioned above, the jet stream was blowing strongly in from the west. The result was more than 150 tornadoes reported throughout the day (Figure below).

The entire region was alerted to the possibility of tornadoes in those late April days. But meteorologists can only predict tornado danger over a very wide region. No one can tell exactly where and when a tornado will touch down. Once a tornado is sighted on radar, its path is predicted and a warning is issued to people in that area. The exact path is unknown because tornado movement is not very predictable.

Fujita Scale

The intensity of tornadoes is measured on the Fujita Scale (see Table below), which assigns a value based on wind speed and damage.

Summary

• A tornado is a whirling funnel of air extending down from a cumulonimbus cloud.
• The Fujita scale measures tornado intensity based on wind speed and damage.
• Tornadoes can only be predicted over a wide region.

Review

1. What causes the tornadoes of Tornado Alley?
2. How does the Fujita scale resemble the scales for assessing earthquake intensity? Which does it most resemble?
3. What circumstances led to the intensity of tornado activity in April 2011?

Explore More

Use this resource to answer the questions that follow.

1. Why do tornadoes form from supercells?
2. What two elements are present in a thunderstorm that will form a tornado?
3. What are winds doing within a thunderstorm to create a tornado?
4. Where does a supercell thunderstorm get its power?
5. What happens when a thunderstorm can finally form in spite of the cap?
6. The meeting of which air masses causes thunderstorms in the spring?
7. Which factor increases the chance of powerful thunderstorms and tornadoes?
8. Why might global warming produce more thunderstorms and tornadoes?