The Wizard of Oz movie in 1939 helped Kansas and the Great Plains become known as part of "Tornado Alley" - the region of the United States often visited in late spring and early summer by dangerous, sometimes violent, tornadoes. Part of the reason why is that the "dryline" -- a front separating moist Gulf of Mexico air from dry air from the Southwest -- often sits across these states, helping spawn tornado-producing thunderstorms. The Glossary of Meteorology published by the American Meteorological Society defines "Tornado Alley" as "a term often used by the media to denote a zone in the Great Plains region of the central United States, often a north-south-oriented region centered on north Texas, Oklahoma, Kansas, and Nebraska, where tornadoes are most frequent."