The Earth's lower atmosphere is composed of a global system of cyclones and anticyclones. In the Northern Hemisphere, cyclonic winds move counterclockwise around centers of low pressure, while anticyclones move clockwise around centers of high pressure. In the Southern Hemisphere, the directions are reversed. The air in a high pressure system is relatively stable, that is, it does not tend to rise. Air in a low pressure system is unstable: it rises and as it does so, becomes more buoyant and continues to rise. Storms are the direct result of the rising air associated with cyclonic instability. Generically, any circular air movement around a low pressure center is a cyclone. Thunderstorms, tornadoes, hurricanes, and dust devils are all intense, localized classes of cyclones.

The central United States, a breeding ground for storm systems, will be used here for a typical cyclone model. The jet stream snakes its way...