A tornado is a spinning column of air that descends from a storm cloud and touches the ground. Certain conditions have to be present for a tornado to form: A cold mass of air has to collide with a warm mass of air.
The National Weather Service defines a tornado as a "violently rotating column of air pendant from a thunderstorm cloud and touching the ground." While tornadoes occur most often in only a few states, every state has experienced them. The states where the most tornadoes touch down are Alabama, Texas, Arkansas, South Dakota, Florida, Oklahoma, Georgia, Nebraska, Illinois, Missouri, Indiana, Mississippi, Iowa, Louisiana and Kansas, according to the EducationWorld website.
A thunderstorm becomes violent when sufficient instability and wind shear are building up in the lower atmosphere, closer to Earth. Unusually warm and humid conditions have to be present in the lower atmosphere, with cooler than usual conditions taking place in the upper atmosphere. When the wind's direction suddenly changes -- wind shear -- and the wind speed increases, the conditions are right for tornado formation. For example, a wind blowing from the south at 15 mph closer to the ground changes to a southwesterly or westerly wind that begins to blow at 50 mph at a higher altitude. This happens before a cold front and low pressure system come into the area.
What makes the tornado spin is the updraft and downdraft winds mixing with the wind shear. The wind shear tilts, forming the familiar upright tornado shape. What makes the tornado more intense is cyclonic air that is twisting counterclockwise around the tornado, according to the WeatherQuestions website.
The average tornado path is 4 miles long and about 400 yards wide. Some tornadoes can be as big as a mile wide and their path can cover 100 miles. Most but not all tornadoes move from the southwest to the northeast. Most tornadoes in the Southern Hemisphere rotate clockwise; Most Northern Hemisphere tornadoes rotate counterclockwise. The winds inside a tornado can spin at speeds up to 300 mph. One-half of all tornadoes form between April and June. Most tornadoes form from 3 to 7 p.m., but they can happen at any time of the day or night, according to the EducationWorld website.