A tornado evolves similarly to a hurricane.
When the earth’s surface is warm enough, the air rises with great speed, causing intense low pressure and increased temperatures. The weather conditions are usually right in summer, though tornadoes have been recorded throughout the year.
Cooler surrounding air with higher pressure rushes in, creating strong convection cells (circulation patterns of warm air rising, cooling, condensing, and sinking). Because strong winds form a spiral, the rushing updrafts begin to spin, forming a funnel.
The funnel of a tornado forms in the sky and grows downward. As it nears the ground, it picks up dust and debris, which allow us to see the swirling winds.
The moving air further decreases the pressure within the tornado, feeding the updrafts and accelerating the wind speed.
Most tornadoes happen in the United States and most of them occur in a path from northern Texas, up through Oklahoma, Kansas, and Nebraska. The frequency of tornadoes here has earned this swath of land the nickname Tornado Alley.