Figure 5.14

Tornado geography. (A) As this tornado risk map shows, tornadoes occur on every continent except Antarctica. (B) The region of the Great Plains that extends roughly from northern Texas to South Dakota is often called Tornado Alley. Warm maritime tropical (mT) air from the Gulf of Mexico, dry continental tropical (cT) air from the interior deserts, and cold continental polar (cP) air from the north frequently meet over the Great Plains. These air masses often stack on top of one another and together provide high humidity, strong instability, and wind shear. These conditions give rise to supercell thunderstorms.