What causes severe thunderstorms and tornadoes? Tornadoes, often accompanied by severe thunderstorms and hail, form in pretty much the same way as severe thunderstorms. In the continental United States, tornadoes usually form in spring and summer, when warm, humid air from the Gulf of Mexico moves across the continent from southeast to northwest and cold, dry air from the Arctic moves across the continent from northwest to southeast. The formation of tornadoes is so chaotic that the direction of rotation of the air just follows whatever conditions start the rotation. This article provides activities that demonstrate how this weather phenomenon works.