Despite all the neccesities needed, tornadoes form in great numbers across a stretch of the United States know as Tornado Alley. In this area, warm, moist Gulf air meets with cool, dry Canadian air during the Spring and early Summer months. This results in a large number of storms during this part of the year. When the warm air meets the cooler air, the warm air rises, and then condenses to form clouds. When an area of warm air rises very quickly, it is know as an updraft.
Updrafts cause the cloud tops to grow higher and higher, sometimes as high as ten miles.
High winds in the upper atmosphere sometimes cause the tops of the storm to be blown to the northeast. This is know as the anvil of the storm. The clouds underneath this anvil are generally free of rain. The rain falls to the northeast of the anvil. Super cells form along an area of instability known as a squall line.