The outbreak of tornadoes that tore across the Gulf and the East Coast Tuesday and Wednesday was unusual for two reasons. For one thing, the severe weather encompassed a significant swath of the country. For another, winter is the least likely time for tornadic thunderstorms.
And yet tornadoes are an expected part of life in the United States—especially in the multi-state area known as Tornado Alley. (Florida, too, sees a disproportionately high number of tornadoes, because of its frequent thunderstorms.) The United States gets more tornadoes, by far, than any other place on the planet. It averages about 1,250 twisters a year. Canada, which sees about 100 tornadoes per year, is a “distant second,” according to the National Centers for Environmental Information.
“The U.S. gets so many tornadoes because, in large part, the presence of the Rocky Mountains and the Gulf of Mexico,” Harold Brooks, a scientist at the National Severe Storms Laboratory, told me in an email. Those features create the conditions for the three key ingredients necessary for the kind of severe thunderstorm that can produce tornadoes:
1. Warm, moist air at low levels
2. Cool, dry air aloft
3. Horizontal winds that increase with height from the ground-up—and change direction, so that they blow from the equator at low levels, and from the west aloft.