Don Gaddie
May 1998

Tornadoes

Tornado (Latin tonare, "to turn"), violent whirling wind, characteristically accompanied by a funnel-shaped cloud extending down from a cumulonimbus cloud (see CLOUD). Commonly known as a twister or cyclone, a tornado can be a few meters to about a kilometer wide where it touches the ground, with an average width of a few hundred meters. It can move over land for distances ranging from short hops to many kilometers, causing great damage wherever it descends. The funnel is made visible by the dust sucked up and by condensation of water droplets in the center of the funnel. The same condensation process makes visible the generally weaker sea-going tornadoes, called waterspouts, that occur most frequently in tropic waters. Most tornadoes spin counterclockwise in the northern hemisphere and clockwise in the southern, but occasional tornadoes reverse this behavior.
The exact mechanisms that cause a tornado to form are still not fully understood, but the funnels are always associated with violent motions in the atmosphere, including strong updrafts and the passage of fronts. They develop within low-pressure areas of high winds; the speed of the funnel winds themselves is often placed at more than 480 km/hr (more than 300 mph), although speeds of more than 800 km/hr (500 mph) have been estimated for extremely strong storms. Damage to property hit by a tornado results both from these winds and from the extremely reduced pressure in the center of the funnel, which causes structures to explode when they are not sufficiently ventilated to adjust rapidly to the pressure difference. The pressure reduction is in keeping with Bernoulli's principle, which states that pressure is reduced as velocity increases.
Tornadoes are most common and strongest in temperate latitudes, and in the U.S. they tend to form most frequently in the early spring; the "tornado season" shifts toward later months with increasing latitude. The number of funnels observed each year can vary greatly in any given region.

Although tornadoes occur throughout the world, including India and Bangladesh, they are most intense and devastating in the United States. Tornadoes can strike at any time of day, but they are much more frequent in the afternoon and evening, after the heat of the day has produced the hot air that is a requirement of a tornadic thunderstorm.

Tornadoes are so common in Tornado Alley because of mountains to the west and the Gulf of Mexico to the south, explains Howard Bluestein, professor of meteorology at University of Oklahoma, and veteran storm chaser. In spring, he says, a strong westerly jet stream flows across the Alley, creating instability and a trough of low pressure that draws in warm, moist air from the Gulf. "Conditions for the supercells [large, powerful thunderstorms] that spawn tornadoes require strong vertical wind shear [changes in wind speed and direction with height] and lots of instability," he says. And that's exactly what happens in Tornado Alley.

The Fujita scale shows the range of violence of tornadoes. An F-5 tornado produces the most violent winds on earth, approaching speeds of 300 miles per hour. (In the Fujita scale, the wind speed is inferred by analyzing the damage, it's not measured directly.)

Tornadoes range in width (as measured by the damage path) from less than 150 feet to more than a mile.

Tornadoes can last from a few minutes to more than an hour.

A tornado can travel along the ground between a few hundred feet to more than 100 miles.

Tornadoes travel along the ground at between 0 and 60 mph.

Other peculiar winds:
Tornadic thunderstorms produce a couple of other bizarre kinds of wind.

A waterspout is a weak (usually) tornado over water. They are most common along the Gulf Coast and southeastern states. In the western United States, they occur with cold fall or late winter storms, when you would least expect a tornado to develop.

A downburst is a downward blowing wind that sometimes comes blasting out of a thunderstorm. The damage looks like tornado damage, since the wind can be as strong as an F2 tornado (!), but debris is blown straight away from a point on the ground. It's not lofted into the air and transported downwind.

Latent heat of condensation (a form of potential energy) is released when the warm air rises and water vapor condenses into liquid water. This latent heat is the energy that liquid water took in when it evaporated to form the water vapor. According to Robert Davies-Jones of the National Severe Storms Laboratory, latent heat is the biggest single source of energy in a thunderstorm. When the released latent heat warms the rising air parcel, the resulting difference in density can push the air up at the extreme velocities needed to create the tornado. The release of latent heat helps cause differences in pressure, which are another form of potential energy. This potential energy is transformed into kinetic energy as increased wind speed. The ultimate source of this wind (kinetic energy) is the sun. In other words, radiation energy was converted to thermal energy, which evaporated water from the oceans. This water contained latent heat energy, which eventually was converted to kinetic energy in the thunderstorm. Thermal energy is transferred between various air bodies within the thunderstorm. Finally, electric energy is released by accumulations of positive and negative charges, causing lightning within the clouds, and from the clouds to the ground. Electric energy is not important to the tornado, but it does attract attention!

Tornadoes release lots of energy, says Davies-Jones. A tornado with wind speeds of 200 mph will release kinetic energy at the rate of 1 billion watts -- about equal to the electricity output of a pair of large nuclear reactors.

But the large thunderstorms that spawn tornadoes are immensely more powerful, releasing latent heat at the rate of 40 trillion watts -- 40,000 times as powerful as the twister, Davies-Jones says.

Let's face it -- many people in tornado country aren't worried about energy flows within tornadoes. If they think much about twisters, it's mainly to wonder why they can't be predicted more accurately.

Poor predictions can cause two problems:
foolish behavior, when people ignore a warning, or hours wasted in shelter, during false alarms.

Tornadoes, unfortunately, are tough to predict. Monster thunderstorms that should produce them don't. Smaller storms that shouldn't, do.

Unfortunately, the tornado-prediction picture seems to be getting worse. The results come from Project VORTEX, the world's largest storm-chasing project.

For better or worse, the only way to get information on these monumental storms is to get up close and personal, says Howard Bluestein, a meteorologist with 19 years storm-chasing experience. "They're fascinating, a violent display of nature that encompass a very small surface area. They're somewhat mysterious, and if you want to know about them, you have to be in the right place at the right time."

Being in the right place at the right time requires a network of vehicles, sensors and radios, and enough graduate students willing to work long hours on the slight chance they'll come face-to-face with a twister.

The work is not extremely dangerous, Bluestein says, if you discount hazards caused by winds, rain, hail and lightning. Storm chasers seldom get blown away by twisters. And the only recruitment problem, he says, is "keeping people away."

According to Robert Davies-Jones, a meteorologist at the National Severe Storms Laboratory in Norman, Okla., recent results from VORTEX show that some tornadoes seem to form much more rapidly -- five to 10 minutes -- than previously thought -- 20 to 30 minutes. "That has obvious warning implications," he says, since you cannot issue a warning until a
tornado has started forming.

A second finding is equally unhelpful to forecasters, he says. Tornadoes can form in small weather patterns that fall between weather stations: "You can get such a small-scale region around a thunderstorm that's favorable for tornado formation, but they may not be detected by the everyday network of detectors."
Most people in tornado alley (defined) have read this all before, but good advice is worth repeating. Here's the bare-bones of self-protection in tornado country.

The first thing you'll need to do is learn the difference between a tornado watch and a tornado warning:

Tornado watch: issued by the National Weather Service when weather conditions make tornadoes likely. Now is the time to remind your family about the safest place in your home. Turn on a radio or television and listen for further announcements. Since tornado prediction is an inexact science, don't expect a lot of warning.

Tornado warning: issued when a tornado has been sighted (visually or on radar). The danger is serious and everyone should go to a safe place, turn on a battery-operated radio and wait for instructions.

What can you do to shelter your home from a direct hit by a tornado? Not much, says Ron Wolfe, a research engineer at the U.S. Forest Products Laboratory, in Madison, Wis. Wolfe, who has investigated storm damage after hurricanes, and the 1984 Barneveld, Wis., tornado, says "The drop in pressure can be quite dramatic, and the house will just blow apart."

Since a tornado only hits any given square mile in tornado alley once in 700 years, he adds, "It's not economically feasible to build a house to resist that kind of wind. That's why you get insurance."

Homes can benefit from simple, cheap measures that will help reduce damage caused by debris impacts or the intense winds around a funnel cloud, Wolfe adds.

Unfortunately, most of these measures must be taken during construction, Wolfe notes. His advice focuses on the roof. If it fails during a storm, heavy rain can soak the insulation and drywall, and destroy the interior.