How Tornadoes Work

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Introduction to How Tornadoes Work

A tornado is one of those amazing, awesome acts of nature that simply leave 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.

Storm Image Gallery

Photo courtesy NOAA
F5 Tornadoes can reach speeds of 261- 318 mph.
See more storm pictures.

In this article, 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

Q: How does a vortex form?

A: A drain's whirlpool, also known as a vortex, forms because of the downdraft 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.

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.

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.

Photo courtesy NOAA
Supercell

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.

Photo courtesy NOAA
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 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 were originally rated on the Fujita Scale, named for its inventor, University of Chicago meteorologist T. Theodore Fujita. Fujita created the scale, based on the wind speed and type of damage caused by a tornado, in 1971. There were six levels on the original scale:

Level	Wind Speed	Possible Damage
F0	40 - 72 mph	Light damage: Tears branches from trees; rips shallow-rooted trees from the ground; can damage sign-posts, traffic signals and chimneys
F1	73 - 112 mph	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
F2	113 - 157 mph	Considerable damage: Well established trees are easily uprooted; mobile homes are dessimated; entire roofs can be ripped off houses; train cars and trucking hauls are knocked over; small objects become dangerous missiles
F3	158 - 206 mph	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
F4	207 - 260 mph	Devastating damage: Houses and other small structures can be razed entirely; automobiles are propelled through the air.
F5	261 - 318 mph	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 damage

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 (DOD). 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 buildling (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, while the estimated highest possible speed is 107 MPH. Meteorologists average of these speeds, meaning the expected wind speed is 89 MPH. An examination of the EF Scale reveals that 89 MPH falls into the EF1 category (86-110 MPH), so the tornado is classifed as an EF1. For more information about the EF scale, see the official NOAA website.

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

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How Tornadoes Work