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.

The United States sees all three of those ingredients. To paraphrase Brooks:

1. Winds out of the South bring warm, moist air from the Gulf at low levels over the eastern half of the United States.

2. Winds out of the West bring air over the Rocky Mountains and High Desert of the Southwest, making that air dry and cool aloft.

3. Because of the Earth’s rotation, the winds are almost always aloft coming out of the West.

All of which means: “When we have winds out of the South near the ground and from the West aloft, the temperature and moisture profiles are right for severe thunderstorms,” Brooks told me. “All that's needed is some mechanism, such as a front, to get a storm started.”

This helps explain why the globe’s middle latitudes, between about 30° and 50° North or South, make for the most favorable environment for tornadogenesis.

“This is the region where cold, polar air meets against warmer, subtropical air, often generating convective precipitation along the collision boundaries,” the NCEI explains on its website. “In addition, air in the midlatitudes often flows at different speeds and directions at different levels of the troposphere, facilitating the development of rotation within a storm cell.”

So while wide expanses of flat land—think: Kansas—are often blamed for playing a role in regions more susceptible to tornadoes, it’s actually how the air flows above that land—and how many supercell thunderstorms form as a result—that makes all the difference.

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