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                           Tornadoes, like hurricanes, are

                         rotating clouds with devastating

winds. Tornadoes are funnels connecting thunder-

storm clouds to the ground. The destroyed area

where funnels touch the ground is usually less

than a mile wide, but may be 50 miles long. Unlike hurricanes, tornadoes do not bring massive amounts of rain and flood waters because they form over land. And yet, tornado winds (up to 300 miles an hour) can completely destroy buildings and trees and kill dozens of people.                                                                                              

So how do tornadoes form?

Learning Objectives

To understand how tornadoes cause natural disasters, you should be able to: 

  • Describe how tornadoes form.

  • Compare and contrast hurricanes and tornadoes.

  • Explain the difference between a tornado watch and a tornado warning.



Enhanced Fujita Tornado Scale—a scale that measures tornado strength based on wind speed and damage.

tornado—a violent, swirling wind that moves over land.

Tornado Alley—an area in the southern plains of central US in which a high number of tornadoes form each year; caused when dry cold air from Canada meets warm moist air from the Gulf of Mexico.

tornado watch—issued by the National Weather Service, a tornado watch means that conditions for tornado formation are possible.

tornado warning—issued by the National Weather Service, a tornado warning means that a tornado has already formed and may soon occur in a specified area.

How Tornadoes Form

   Tornadoes are rapidly rotating masses of air that touch the ground. To form, they need rising air (warm and humid at low levels), and winds blowing in different directions at higher elevations.

   Dense cool air is heavier than warm air, but sometimes strong winds push cold air on top of warmer air to create a thunderstorm. Rotation of the column of air starts in the thunderstorm where two air masses traveling in different directions meet. As the rising air cools, the cooler air flows downward, and it forms a spinning funnel. As more warm air rises, it brings more energy that causes the funnel to spin more and more rapidly. When the funnel wind speed reaches 65 mph and the funnel extends to the ground, a tornado is formed.












   In the USA, most of the 1200 tornadoes that happen each year form in the South and center of the country in an area called Tornado Alley. That is where warm, humid air from the Gulf of Mexico collides with cool, dry air from Canada and from the Pacific coast.

   Look at the map above. The red and orange area running from Texas to the Dakotas is Tornado Alley, but notice that tornadoes also occur east of the Alley. Florida has a lot of tornadoes because it is hit by hurricanes that often create tornadoes over land.

A Tornado Damage Scale

      Like hurricanes, there is a scale to measure tornado strength. The Enhanced Fujita Tornado Scale is based on damage. This is because it is very difficult to measure tornado wind speed – the instruments are often destroyed or blown away. The wind speed is estimated based on studies where instruments survived and from measurements by weather radar.















Tornado Safety

   It used to be that a tornado could appear without warning.  Now, the National Oceanic and Atmospheric Agency (NOAA) provides advance alerts on radio, TV and online about tornadoes and other dangerous events (flash floods, hail storms, blizzards, etc). NOAA provides Tornado Watch and Tornado Warning alerts.















Did you know?
Tornadoes have been recorded in all 50 states.

   If you are outside when a tornado warning occurs, go inside the nearest strong building. Shelter in the basement or in a windowless room (bathroom or closet) on the lowest floor. Get under a heavy piece of furniture (a table or a desk) and crouch down with your arms covering your head.

   Flying debris is the main cause of injuries from tornadoes. Another cause is being rolled across the ground by high winds. But if you are trapped outside, laying on the ground is safer than being in a car because cars are larger and are more likely to rolled and destroyed by powerful winds.


Check Your Understanding:

1. Describe how tornadoes form.


2. In the US, where do most tornadoes form?


3. Explain how tornado strength is measured.


4. Compare and contrast hurricanes and tornadoes.


5. Explain the difference between a tornado watch and a tornado warning.

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Image of a tall tornado funnel reaching the ground from a dark sky
Map showing the number of tornadoes a year in the US in varying shades of yellow and red
Table showing the categories of tornado wind speeds and strengths
Chart detailing the difference between a tornado watch and warning on a background showing a tornado touching ground
NDH logo.jpg
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