North America is the tornado capital of the world and the majority of these twisters occur in the Southeastern and Central U.S. Tornadoes may occur during any month of the year but their numbers peak from March through June.
The largest and most powerful tornadoes tend to develop in a broad swath from central Texas to Nebraska, a zone dubbed "Tornado Alley." This region is prone to the formation of super cell thunderstorms: giant, rotating storms with cloudtops near 50,000 feet. These super cells usually form along the "dry line," which trails southward from the front's zone of low pressure. At this line, dry air from the west clashes with warm, moist air from the Gulf of Mexico; just east of the line, the dry air temporarily overrides and caps the muggy air at the surface. Eventually, this cap breaks down and the warm, unstable air rushes into the cold, upper atmosphere, where the jet stream injects more energy and triggers rotation: a super cell is born.
Moving northeastward in advance of the front, these massive storms often spawn tornadoes at their trailing, southwestern edge. Like hurricanes, super cell thunderstorms feed on the warm, moist air that flows up through their center; should they encounter rain-cooled air left behind by other storms, they begin to dissipate. For that reason, the loner storms tend to be more powerful and more dangerous; storm chasers know that these solitary monsters provide the best opportunity for close encounters with twisters.
