Mystery of twin tornadoes :

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The twin tornadoes that leveled the small city of Pilger, Nebraska on Monday, killing two and injuring nearly two dozen, were a horrifying, relatively rare phenomenon. One of the twisters was once rated an EF-4 on the Enhanced Fujita Tornado Damage Scale, with winds as high as 200 miles per hour.

As strong and photogenic as they were, however, the tornadoes were not somewhere close to unprecedented, based on an examination of tornado research and interviews with extreme weather experts.

Tornadoes in general are uncommon and extraordinarily complicated phenomena, occurring between a few hundred to up to 2,000 times per year in the U.S. There are many things scientists nevertheless don’t know about tornadoes. For example, they still don’t know exactly why some severe, rotating thunderstorms, acknowledged as supercells, produce tornadoes while other nearly identical storms do not.

They additionally lack a complete inventory of the types of possible tornadoes, with storm chasers documenting all varieties of strange behavior, from intense “suction vortices” that killed several well-known storm chasers in 2013, to the formation of more than one tornadoes over time from the same parent thunderstorm.

In fact, the more one delves into the classification of tornadoes and the history of twister research, the more dizzy one becomes, with twister after twister that departs from the classical definition in popular tradition of a single funnel extending from a parent thunderstorm to the ground.

So, the first rule of tornadoes is that no two tornadoes are exactly alike.

As Karen Kosiba, a researcher with the Center for Severe Weather Research in Boulder, Colorado, told Mashable, “There’s so many different constructions and so many different configurations that tornadoes can be in.”

Two tornadoes approach Pilger, Nebraska, Monday June 16, 2014.

“This is just one of those that passed off to be visually spectacular,” she said.

The basics of tornado formation
Tornadoes form when a severe thunderstorm with a constantly rotating updraft tilts a horizontally rotating column of air upward into the vertical, beneath a persistent area of rotation known as a mesocyclone. Conditions have to be simply right for a tornado to form, grow, and intensify. For example, wind shear — the name for the phenomenon where wind blows at various speeds and in different directions at different altitudes, causing rotation between the layers — wants to be significant. So, too, does atmospheric instability, which encourages the formation of intense thunderstorms and aids tornado formation as well.

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