Tornadoes are one of those amazing, awesome acts of nature that simply leaves you dumbfounded -- a huge, swirling, 200-mph beast of a storm that appears to have a mind of its own. You have to actually see one with your own eyes to believe it. In certain places, tornadoes appear with amazing regularity. That's why we see them in the news all the time.


Photo courtesy NOAA

In this edition of HowStuffWorks, we will take a look at tornadoes to learn what they are, how they form and just how powerful they can be.

Tornadoes and Your Bathtub
If you have ever seen a whirlpool form in your bathtub, sink or toilet when the water is draining, you have seen 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 the vortex forms.


Why should the water start rotating? There are lots of explanations, but here is one way to think about it (and this way happens to apply to black holes as well as it does to drains). Imagine you are a particle in the water, and you are being pulled toward the suction that the drain creates. You are accelerating toward the point of suction. However, because of your previous momentum, the number of other particles getting sucked toward the point and other factors, chances are that you are going to be off to one side of the point of suction when you arrive. That deflection sets you up 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 of the other particles as they arrive. A very strong spiraling tendency is created. Eventually, there is enough spiraling energy to create a vortex.

Given that you see vortexes all the time in tubs and sinks, it is obviously a fairly common phenomenon. In a tornado, the same sort of thing happens, except with air instead of water.

Tornadoes and Thunderstorms
With a tornado there is no drain. Instead, there is a thunderstorm cloud. A typical thunderstorm cloud can accumulate a huge amount of energy. If the conditions are right, this energy creates a huge updraft into the cloud. But where does the energy come from?


Photo courtesy NOAA

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. According to Encyclopedia Britannica:

    For every gram of water condensed, about 600 calories of heat are made available. When the water freezes in the upper parts of the cloud, another 80 calories of heat per gram of water are released. This energy goes to increase the temperature of the updraft and, in part, is converted to kinetic energy of upward and downward air movement. If the quantity of water that is condensed in and subsequently precipitated from a cloud is known, then the total energy of a thunderstorm can be calculated. In an average thunderstorm, the energy released amounts to about 10,000,000 kilowatt-hours, which is equivalent to a 20-kiloton nuclear warhead. A large, severe thunderstorm might be 10 to 100 times more energetic.
In supercell thunderstorms, the updrafts are particularly strong (see the links at the end of this article for information on supercells). If they are strong enough, a vortex of air can form just like a vortex of water forms in a sink. An air vortex under a thunderstorm cloud is a tornado.


A tornado snakes down out of the thundercloud and eventually touches ground. When it does, it picks up a swirling cloud of dust and debris.

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 are not 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.

The tornado follows a path that is controlled by the path of its parent thundercloud, and it will often appear to hop. The hops occur when the vortex is disturbed. You have 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 form and collapse along its path.

Tornado Ratings
Tornadoes are rated on what is called the Fujita Scale, named for the inventor. There are six levels in this scale:

Level
Wind Speed
F0
40 - 72 mph
F1
73 - 112 mph
F2
113 - 157 mph
F3
158 - 206 mph
F4
207 - 260 mph
F5
261 - 318 mph

See this page for damage descriptions.

For more information on tornadoes and related topics, check out the links on the next page.

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