The thunderstorm that tore through New York City on Wednesday unloaded 3 inches of rain in an hour and spawned a tornado that ripped the roofs off dozens of Brooklyn homes. It was the borough's first twister since the National Weather Service started keeping reliable records in 1889. Why doesn't the Northeast get more tornadoes?
Because the Appalachians aren't high enough, and the Gulf Stream isn't warm enough. Tornadoes require precise conditions to form. There has to be warm, moist air close to the ground and cold, dry air about 10,000 feet or higher in the atmosphere. When the two collide, you get a thunderstorm. The final ingredient for a twister is wind that changes directions and picks up speed with altitude. In the Northeast, the Gulf Stream supplies some warm air and the Appalachians dispatch some cool air to the coast. Neither effect is very strong, though, which explains why tornadoes in this neighborhood are rare. (Occasionally, ocean breezes blowing westward in the afternoon make tornadoes possible in Boston and Long Island.) According to a map from NOAA, a spot in New York City is likely to be struck by an F-2 tornado just once every 20,000 to 50,000 years.
The so-called Tornado Alley that stretches across the Midwest, from northern Texas to South Dakota, results from near ideal conditions for tornado formation. Winds ferry heat and moisture from the Gulf of Mexico, while the high, expansive Rockies drive lots of cold, dry air toward the plains. To get over the top of the mountains, the mass of air squeezes into a short column; coming down the other side, the column stretches to fill the extra space, which causes the air to start spinning counterclockwise like a storm system. The world's No. 1 tornado hot spot lies about 50 miles south of Oklahoma City; an F-2 twister should hit the same spot there once every 3,800 years. Sometimes they return more frequently. A tornado assaulted Codell, Kan., on May 20, 1916, then again on the same day for each of the next two years. In the tornado outbreak of 1974, 148 twisters touched down over 16 hours in states from Michigan to Georgia. More than 300 people died.
Tornadoes are mostly an American phenomenon. The United States has about 1,000 tornadoes per year, more than any other country. But other regions are also prone to twisters. The cold-hot combination of the Andes Mountains and the Amazon jungle makes Argentina, Uruguay, and southern Brazil similar to the American Midwest. Bangladesh and southern China are also likely candidates for tornadoes, and strong tornadoes have also been reported in South Africa, Australia, and Northern and Central Europe. Poland and Iceland were both hit in recent weeks.
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Explainer thanks Harold Brooks and Daniel McCarthy of National Oceanic and Atmospheric Administration and Frederick Carr of the University of Oklahoma.