June 17, 2014

A deadly double tornado struck Nebraska yesterday, causing 2 deaths and leaving at least 16 people injured. The two tornadoes touched down at the same time and traveled with about a mile (1.6 kilometers) between them. The small farming community of Pilger, Nebraska, about 60 miles (97 kilometers) southwest of Sioux City, Iowa, sustained heavy damage from the twisters. Monday’s storm system also brought damaging winds and rains to other Midwestern States, including Iowa, Minnesota, South Dakota, and Wisconsin. One area of South Dakota received more than a half-inch (1.27 centimeters) of rain in fewer than 3 minutes, flooding roads and interstates.

In addition to bringing high winds and rains, large, powerful thunderstorms can form deadly tornadoes. Such storms are called supercells. A supercell contains a rapidly rotating column of air called a mesocyclone. A smaller, swiftly rotating column of air called a funnel cloud can develop within the mesocyclone. If the funnel cloud reaches the ground, it becomes a tornado.

A tornado is a rapidly rotating column of air that can develop under a large, anvil-shaped thundercloud. Almost all tornadoes in the Northern Hemisphere rotate as shown in the diagram—counterclockwise when viewed from above (World Book illustration by Bruce Kerr).

Weather experts state that it is not uncommon for more than one tornado to form from a supercell. Usually, however, the first tornado shrinks and disappears as the second forms. Or, when two twisters form and both remain, there is usually one large, strong tornado and one smaller tornado. It is less common for the primary twister to keep going strong when a new twister forms, producing two tornadoes of the same strength at the same time. Nevertheless, that is what hit Nebraska Monday evening.

Additional World Book articles:

• Tornado Alley
• Twisted: More Terrible Tornadoes (a Special Report)

April 4, 2012

Rare twin supercells unleashed at least 18 tornadoes on the Dallas-Forth Worth metropolitan region. (A supercell is a massive low-pressure center characterized by a very large cumulonimbus cloud and long-lasting thunderstorm, often producing numerous and violent tornadoes.) The National Weather Service characterized the storms as among the most destructive in the area’s history. Twisters flung several massive semi-tractor trailers into the air and tossed a school bus across a road and into a diner. Hundreds of houses were severely damaged or leveled in the Dallas suburbs of Arlington, Lancaster, and Forney. In Arlington, an entire wing of a nursing home collapsed, forcing the evacuation of dozens of residents. The storms left thousands of households without electric power. Miraculously, there were no fatalities and relatively few injuries.

A supercell thunderstorm is a violent storm dominated by a single gigantic cell--a weather system made up of storm clouds and the winds associated with them. Rain and hail may fall for hours. (National Center for Atmospheric Research/University Corporation for Atmospheric Research/National Science Foundation)

At the Dallas-Ft. Worth International Airport, all flights were canceled, stranding thousands of passengers. Baseball-size hail pounded planes on the tarmac waiting for take-off. “The noise of ice . . . hitting the aluminum exterior of a 757 was as deafening as it was frightening,” noted one passenger.

Although the tornado season has just started in the United States, meteorologists point out that there have already been dozens of destructive twisters from Illinois to Texas.

Additional World Book articles:

• Safety (During a tornado)
• Storm