Twisters, or tornadoes, form during super cell thunderstorms in which a column of air rotates, according to The National Center of Atmospheric Research. The air increases speed in an updraft of the thunderstorm, and a funnel cloud forms. Once the funnel of wind hits the ground, it is a tornado.
The possibility of a tornado formation occurs before a thunderstorm produces effects on the ground, explains the National Weather Service. The rotation in the lower atmosphere occurs as the wind changes before a large thunderstorm. Updrafts within the lower atmosphere change the direction of air to a vertical wind. The thunderstorm also produces rain and hail before the tornado typically touches down, details The National Center of Atmospheric Research. The Enhanced Fujita Scale measures the strength of each tornado by factors including wind speed and the level of destruction.
Tornadoes occur anywhere in the United States, warns National Atlas. Tornado Alley is a concentrated area of tornadic activity in the United States. The area stretches from Iowa to Texas and has the highest number of tornadoes each year in the United States, explains the National Severe Storms Laboratory. Tornado season varies by region, but the most active time of the year is from early spring through July.