Southern Tornado Outbreak: How do tornadoes form? - KMSP-TV

Southern Tornado Outbreak: How do tornadoes form?

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Meteorologist Steve Frazier Meteorologist Steve Frazier
The good thing about being on the northern side of the latest storm system to wreak havoc on the country is the fact that we are only dealing with the rains, and not the severe weather. We will however get our turn as the warmer weather seasons progress.

The weather setup down south this week has been the perfect storm of ingredients for tornadoes to form.  Since knowledge is power, as we get deeper into our severe weather season, here is a recap of just how tornadoes form.


Thunderstorms develop easiest when we have cool air aloft and warm moist air below. The exact scenario is playing out this week across the country as our slow moving low continues to bring together these two air masses.

The Supercell Thunderstorm that eventually gives birth to a tornado needs these three ingredients to form.

Moisture - Moisture is necessary to form the cloud and rain. A big source of moisture in the United States comes from the Gulf of Mexico.

Instability - Warm air is less dense (lighter) than cold air. The sun warms the ground and the ground warms the air above it. Evaporation of moisture from the ground or bodies of water increases the humidity. The warming of the air and the increase in humidity cause the air mass to destabilize. If there is cooler, drier air above, the tendency would be for the air to want to overturn with the cooler air that is sinking and the warmer air that is rising. This is instability.

Lift - This is the trigger that starts air rising and focuses storms. A few examples include air colliding with a front. A front is a boundary between two different air masses. Where the air masses collide, the less dense air (warmer or more humid) will rise over the other. The cool outflow from a thunderstorm forms a "gust front" which may in turn cause the development of new thunderstorms. These are all sources of lift. The jet stream can greatly increase lift by providing greater upward motion. You can see that the jet stream this week is right over the tornado outbreak in the south.

The warm moist air rises, carrying with it lots of water. In fact a strong updraft can carry 8000 tons of water into the atmosphere every minute. Simply put, the moisture travels to a height where it cools and condenses, forming precipitation. The falling water brings down cool air from above. Also, the evaporation of the water cools the air, and the cooler denser air accelerates this downdraft.

This downdraft becomes more serious when there is wind shear. Wind shear is simply winds blowing in different directions.

This wind shear causes rotating columns of air and the strong down draft can pull that rotation to the ground, forming the tornado. The downdraft accelerates the rotation, just like a figure skater doing a spin. Its this rotating air that eventually reaches the ground forming a tornado.

If the rotating air doesn't’t make contact with the ground, this is known as a funnel cloud.

Now you know a bit more about what is going on when the severe weather strikes. 

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