What Are Tornadoes? - Definition, Causes & The Enhanced Fujita Scale

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  • 0:06 What Are Tornadoes?
  • 1:00 How Do Tornadoes Form?
  • 2:35 Where Do Tornadoes Occur?
  • 3:33 Enhanced Fujita Scale
  • 4:42 Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Tornadoes are dangerous storms that affect millions of people. In this video lesson, you will learn how and where they form, as well as how scientists classify tornadoes using the Enhanced Fujita Scale.

What Are Tornadoes?

The air gets thick and still. The wind stops. The sky turns green. And then you hear the siren warning you to take cover. If you've ever lived in a place where tornadoes occur, this is a familiar sequence of events. Tornadoes are spinning funnel clouds produced by thunderstorm clouds.

Acting like an arm that reaches down from the thunderstorm cloud, these violent rotating funnels extend from sky to ground. In fact, in order for a funnel cloud to be officially classified as a tornado, it has to be touching both the source cloud and the ground.

Tornadoes are dangerous events. These storms can be several miles wide and have wind speeds as fast as 300 mph! They have indiscriminate and unpredictable travel routes and act like giant vacuum cleaners picking up everything in their paths.

Tornadoes form from thunderstorm clouds, which form when warm, wet air rises very quickly. As it rises, the cool air condenses into a large cumulus cloud. Like adding fuel to a fire, more rising warm air builds the cloud larger and larger. Eventually, the water in the cloud falls back to the ground as rain. As it falls, it brings some of that cooler air back down with it.

The warm air rising is called an updraft (because it's traveling upward), and the cool sinking air is called a downdraft (since it's falling back down to the ground). When these two work together like this, they create a storm cell.

Some thunderstorms stay like this, but some grow even larger and become supercells. But a tornado doesn't begin to form until the air starts to spin. Scientists are not exactly sure how this happens, but it seems that one way this is possible is when winds at different altitudes within the supercell travel at different speeds.

You can try this at home and see how it works. Take a round object, like a ball, and put one hand on each side. Now move both hands forward at the same speed - the ball moves, but it doesn't spin. Now move both hands forward, but have one hand move faster than the other - the ball moves and spins! This is what scientists believe happens in the supercell that creates the spinning funnel cloud. And just like the rain brings down cool air as it heads toward the ground, it also brings down the funnel cloud and turns it into a tornado.

Where Do Tornadoes Occur?

Remember Dorothy and her poor dog, Toto? They were ripped from Kansas in a wild tornado and taken far from home. While it's doubtful that you'd actually get taken to Oz if you were caught in a tornado, Kansas is, in fact, well-known for its high frequency of this type of storm. So are other states in the area, such as Texas, Oklahoma, Nebraska, South Dakota, Missouri and Iowa. Collectively, this area is known as Tornado Alley because tornadoes are so frequent, especially in the summer.

Tornado Alley exists because the Great Plains get very hot in the summer. Updrafts are all too common, which as we now know creates thunderstorms and supercells. Florida is also well-known for its frequent tornadoes for this very same reason. Florida has more thunderstorms than any other state in the country, and all that warm, moist air creates the perfect conditions for supercells and tornadoes to form.

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