Myths are full of fantastic and destructive creatures. If it's not a city-leveling angel, then it's giants sauntering vengefully into unsuspecting towns. In reality, all the calamities we're liable to encounter are due to natural phenomena and human will. But of all the destructive powers in our world, none resembles the ferocity and form of those mythic monsters quite like tornadoes. These storms descend like a dagger from the clouds. They tower over the tallest buildings like titans. And when they lash out at their surroundings, they often seem to act with malicious, mindful intent.
Set aside fear and superstition, and you're still faced with one of the most awesome sights in the natural world. These twisting storm columns can reach wind speeds of 318 mph (512 kph) and measure miles across, scarring the Earth and decimating homes and buildings in the process. Yet, in some parts of the world, these powerful storms are a regular occurrence. The United States alone experiences more than 1,000 tornadoes a year, and the storms have been reported on every continent except Antarctica [source: Tarbuck].
While most storms are weak and occur in sparsely populated areas, tornadoes have been known to hit large metropolitan areas, and they have inflicted heavy casualties on many towns and cities. In 1925, the infamous U.S. tristate twister hit parts of Missouri, Illinois and Indiana, claiming 695 lives.
In this article, we'll examine how a tornado's vortex works, take a close look at tornado formation and discuss the system we use to classify them.
What Your Bathtub Can Teach You About Tornadoes
If you've ever watched a whirlpool form in your bathtub or sink while draining the water, then you've witnessed the fundamentals of a tornado at work. A drain's whirlpool, also known as a vortex, forms because of the downdraft that the drain creates in the body of water. The downward flow of the water into the drain begins to rotate, and as the rotation speeds up, a vortex forms.
Why does the water start rotating? There are many explanations, but here's one way to think about it. Imagine yourself as a particle in the water, suddenly pulled toward the suction that the drain creates. At first, you'd find yourself accelerating toward the drain. Then, quite literally, there's a twist. Because of your previous momentum and the number of other particles rushing toward the drain at the same time, chances are that you're going to be pushed off to one side of the point of suction when you arrive. That deflection sets you on a spiraling path into the point of suction, like a moth spiraling in toward a light. Once the spiral has started in one direction, it tends to influence all the other particles as they arrive. A very strong spiraling tendency is created. Eventually, there's enough spiraling energy to create a vortex.
Vortices are obviously a common phenomenon. After all, you see them in tubs and sinks all the time. Small dust devils sometimes form when winds flow over hot deserts, and wildfires have been known to produce climbing vortices of flame and ash called fire whirls. Scientists have even observed dust devils on Mars and spotted solar tornadoes whipping out from the sun.
In a tornado, the same sort of thing happens as with our bathtub example, except with air instead of water. A great deal of the Earth's wind patterns are dictated by low-pressure centers, which draw in cooler, high-pressure air from the surrounding area. This airflow pushes the low-pressure air up to higher altitudes, but then the air heats up and is pushed upward as well by all the air behind it. The air pressure inside a tornado is as much as 10 percent lower than that of the surrounding air, causing the surrounding air to rush in even faster.
How do weather conditions pull the plug on atmospheric conditions? Skip to the next page to find out how tornadoes form.
Tornadoes and Thunderstorms
Tornadoes don't just pop into existence -- they develop out of thunderstorms, where there's already a steady, upward flow of warm, low-pressure air to get things started. It's kind of like when a rock concert erupts into a riot. Conditions were already volatile; they merely escalated into something even more dangerous.
Thunderstorms themselves form like many other clouds: A warm, moist air mass rises and cools, causing the water vapor to condense into clouds. However, if the updraft continues, this cloud mass will continue to grow and rise 40,000 feet (12,192 m) or more up into the troposphere, the bottommost layer of the atmosphere that we live in. A typical thunderstorm cloud can accumulate an enormous amount of energy. If the conditions are right, this energy creates a huge updraft into the cloud, but where does the energy come from?
Clouds are formed when water vapor condenses in the air. This change in physical state releases heat, and heat is a form of energy. A good deal of a thunderstorm's energy is a result of the condensation that forms the cloud. Every gram of water condensed results in about 600 calories of heat -- and another 80 calories of heat per gram of water results from freezing in the upper atmosphere. This energy increases the updraft temperature, as well as the kinetic energy of upward and downward air movement. The average thunderstorm releases around 10,000,000 kilowatt-hours of energy -- the equivalent of a 20-kiloton nuclear warhead [source: Britannica].
In supercell thunderstorms, the updrafts are particularly strong. If they are strong enough, a vortex of air can develop just like a vortex of water forms in a sink. This precursor to the tornado is called a mesocyclone, and is typically 2 to 6 miles (3 to 10 kilometers) wide. One a mesocyclone forms, there's a roughly 50 percent chance that the storm will escalate into a tornado in around 30 minutes.
Some tornadoes consist of a single vortex, but other times multiple suction vortices revolve around a tornado's center. These storms-within-a-storm may be smaller, with a diameter of around 30 feet (9 meters), but they experience extremely powerful rotation speeds.
The tornado reaches down out of a thundercloud as a huge, swirling rope of air. Wind speeds in the range of 200 to 300 mph (322 to 483 kph) aren't uncommon. If the vortex touches ground, the speed of the whirling wind (as well as the updraft and the pressure differences) can cause tremendous damage, tearing apart homes and flinging potentially lethal debris.
The tornado follows a path that is controlled by the route of its parent thundercloud, and it will often appear to hop. The hops occur when the vortex is disturbed. You've probably seen that it is easy to disturb a vortex in the tub, but then it will reform. The same thing can happen to a tornado's vortex, causing it to collapse and reform along its path.
Smaller tornadoes may only thrive for a matter of minutes, covering less than a mile of ground. Larger storms, however, can remain on the ground for hours, covering more than 90 miles (150 km) and inflicting near continuous damage along the way.
At this point, you might be wondering just how tornadoes eventually dissipate. Scientists still debate exactly how these deadly storms die, but one of the prime suspects is none other than the parent thunderstorm: the rotating mesocyclone. Tornadoes need instability and rotation. Disrupt the airflow, take away its moisture or destroy its unstable balance of hot and cold air, and it can't function. Often, a tornado will die because the cold outflow of air from falling precipitation upsets the balance.
How do we categorize these different storms? Skip to the next page to find out.
Tornadoes are among the most dangerous storms on Earth and, as meteorologists strive to protect vulnerable populations through early warning, it helps to classify storms by severity and potential damage. Tornadoes were originally rated on the Fujita Scale, named for its inventor, University of Chicago meteorologist T. Theodore Fujita. The meteorologist created the scale in 1971 based on the wind speed and type of damage caused by a tornado. There were six levels on the original scale.
- Wind Speed: 40-72 mph (64-116 kph)
- Light damage: Tears branches from trees; rips shallow-rooted trees from the ground; can damage signposts, traffic signals and chimneys
- Wind Speed: 73 - 112 mph (117 - 180 kph)
- Moderate damage: Roofing materials and vinyl siding can be displaced; mobile homes are highly vulnerable and can easily be knocked from the foundation or toppled; motorists can be sent careening off road and possibly flipped over
- Wind Speed: 113 - 157 mph (181 - 253 kph)
- Considerable damage: Well-established trees are easily uprooted; mobile homes are decimated; entire roofs can be ripped off houses; train cars and trucking hauls are knocked over; small objects become dangerous missiles
- Wind Speed: 158 - 206 mph (254 - 332 kph)
- Severe damage: Forests are destroyed as a majority of trees are ripped from the ground; entire trains are derailed and knocked over; walls and roofs are torn from houses
- Wind Speed: 207 - 260 mph (333 - 418 kph)
- Devastating damage: Houses and other small structures can be razed entirely; automobiles are propelled through the air
- Wind Speed: 261 - 318 mph (419 - 512 kph)
- Incredible damage: Cars become projectiles as they are hurled through the air; entire houses are completely destroyed after being ripped from the foundation and sent tumbling into the distance; steel-reinforced concrete structures can be seriously damaged [source: NOAA]
In February 2007, the Fujita Scale was replaced by the Enhanced Fujita Scale. The new "EF" scale is similar to its predecessor. It classifies tornadoes into six different categories (EF0 through EF5 instead of F0 through F5). Where the EF scale differs, however, is in the number of criteria used to assess a tornado's level of damage. First, there are damage indicators -- objects that can be damaged in the tornado. These are classified from 1 (small barns) to 28 (softwood trees). Each damage indicator can also experience varying degrees of damage (DODs). Each DOD corresponds to estimated wind speeds.
For example, a motel has 10 degrees of damage, ranging from broken windows (3) to the collapse of most of the roof (6) to complete destruction of the building (10). If a motel's windows are broken, but it doesn't sustain more extensive damage, the estimated lowest possible wind speed is 74 mph (119 kph), while the estimated highest possible speed is 107 mph (172 kph). Meteorologists average these speeds, meaning the expected wind speed is 89 mph (143 kph). An examination of the EF Scale reveals that 89 mph falls into the EF1 category, so the tornado is classified as an EF1. For more information about the EF scale, see the official NOAA Web site.
Explore the links on the next page to learn even more about tornadoes and other weather anomalies.
Frogs! Fish! Birds! They've all been said to appear from clouds. Learn about 10 times the sky rained something other than water at HowStuffWorks.
- What is it like in the eye of a tornado?
- Is there really "a calm before a storm"?
- How Storm Chasers Work
- How the Totable Tornado Observatory Worked
- How the Tornado Intercept Vehicle Works
- 15 Tornado Safety Tips
- How Hurricanes Work
- 5 Most Destructive Storms
- How Weather Works
- How Weather Alerts Work
- How Floods Work
- How Wildfires Work
More Great Links
- Davis, T. Neil. "Dust Devils Article #227." Alaska Science Forum. June 2, 1978. (Sept. 26, 2008)http://www.gi.alaska.edu/ScienceForum/ASF2/227.html
- Edwards, Roger. "The Online Tornado FAQ." NOAA. May 26, 2008. (Oct. 2, 2008)http://www.spc.noaa.gov/faq/tornado/
- "Surprises From SOHO Include Tornadoes On The Sun." Science Daily. April 20, 1998. (Sept. 26, 2008) http://www.sciencedaily.com/releases/1998/04/980430083400.htm
- Swanson, Bob and Doyle Rice. "Fire whirl erupts during California blaze." USA Today. July 13, 2006. (Sept. 26, 2008)http://blogs.usatoday.com/weather/2006/07/fire_whirl_erup.html
- Tarbuck, Edward and Frederick Lutgens. "Earth Science: Eleventh Edition." Pearson Prentice Hall. 2006.
- "Tornado." Britannica Online Encyclopædia. 2008. (Sept. 26, 2008)http://www.britannica.com/EBchecked/topic/599941/tornado
- "Tornado Science, Facts and History." Live Science. (Sept. 26, 2008)http://www.livescience.com/environment/050322_tornado_season.html