Atmospheric conditions typical during tornado formation.
How a Tornado Forms
A tornado begins in a severe thunderstorm called a supercell. Scientists aren't exactly sure why, but air coming into the storm begins to swirl and forms a funnel. The air inside the funnel spins so fast it pulls in more air--and objects! The funnel is kind of like a big vacuum cleaner. The air pressure is very low inside the funnel, just like pressure is low inside the eye of a hurricane. Only in a tornado, the pressure is a lot lower--lower than any other place on earth. Tornadoes can form when the atmosphere is unstable. One other thing that's needed is a front, a place where warm moist air meets cold dry air. This happens a lot in the Great Plains of the United States which is why it's called Tornado Alley.
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