Atmosphere And Weather

Tornado Alley Explained



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A quirk of geology makes the Midwestern United States a perfect spawning ground for powerful tornados. The unique arrangement of North America's mountain ranges creates the world's most prolific source of severe weather, commonly known as "Tornado Alley."

Most of the continents on Earth have mountain chains that run more-or-less east to west: Europe's Alps, the Atlas Mountains in Africa, and Asia's mighty Himalaya. East-west mountain ranges tend to dampen severe weather, because they keep colder polar air-masses separated from warmer tropical air.

North America, however, has only north-south mountain ranges. Together, the Appalachian Mountains in the east and the Rocky Mountains in the west form an enormous funnel that directs cold, dry air southwards from Canada, and warm, moist air northwards from the Gulf of Mexico. The cold Canadian air comes in high in the atmosphere, at 10,000 feet or more in altitude. The denser, humid Gulf air slides in underneath. When the wind starts whipping across the Plains, as it is prone to do, the different air masses begin to mix and churn, with dramatic (and often deadly) results.

During the warm months of each year, towering cumulonimbus clouds blossom in the skies over Texas, Oklahoma, Kansas, Nebraska, the Dakotas, and as far east as Ohio and Arkansas, bringing massive thunderstorms, flash floods, hail... and tornados.

While every state in the Union has experienced at least one tornado, those that strike Tornado Alley are notable not only for their frequency, but also their ferocity. The May 22, 2004 tornado that struck Hallam, Nebraska was an astonishing 2 1/2 miles wide. The infamous Tri-State Tornado that scrawled a message of devastation across parts of Missouri, Illinois, and Indiana on March 18, 1925 was in contact with the ground for an unbelievable 219 miles!

The U.S. experiences around 1,200 tornados per year, with the vast majority occurring in Tornado Alley. This region is home to nearly all of the storms rated F3, F4, or F5 on the Fujita Scale. An F3 tornado has sustained winds of 158-206 miles per hour, strong enough to tear off roofs, destroy mobile homes, and uproot large trees. An F5, or "Finger of God," has winds of 261-318 miles per hour, and can level whole neighborhoods down to the foundations, and toss cars around like toys.

These most powerful storms are responsible for at least 75% of tornado deaths, and they are confined almost entirely to Tornado Alley. The number of deaths due to tornados does vary widely from year to year, though, depending on the number of F3 and stronger storms, as well as how many densely-populated areas are struck. In 1986, only 15 Americans were killed by tornados. 1953, however, was a particularly deadly tornado year, with 519 fatalities. Although the population continues to increase across Tornado Alley, fatalities have declined fairly steadily over the past 50 years, as tornado forecasting, detection, and warnings have improved.

The arrangement of North America's mountain ranges makes the Midwestern U.S. a perfect place for severe thunderstorms and tornados to develop. Modern weather forecasting and heightened public awareness have combined, however, to make Tornado Alley a safer place to live than it was in decades past.

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