"Toto, I've a feeling we're not in Kansas anymore," Dorothy said in The Wizard of Oz after a tornado -- that most American of extreme weather events -- deposited her into a land over the rainbow. In a typical year some 1,300 real tornadoes rip across the U.S., hitting the Midwest's "Tornado Alley" far more frequently and severely than any other region on earth. The most fearsome of tornadoes -- the deadly Wicked-Witch-of-the-East-slaying, enhanced-Fujita-scale-5 (EF-5) twisters, which contain top winds exceeding 300 kilometers per hour -- have hit the U.S. 59 times since 1953. Only one EF-5 has ever been documented outside the U.S. -- in Manitoba, Canada, in 2007.

Tornadoes are just one type of extreme storm that Scientific American magazine has long documented. Its detailed coverage of tornadoes, for example, began as early as 1897 with the publication of "The Varying Power of Tornadoes and Their Frequency," continued in 1927 with "Our Worst Storm, the Tornado," and began to delve deeply into physics behind tornado formation in the 1958 article "Tornadoes," by NASA meteorologist Morris Tepper. From some of these early investigations, we learn that the key ingredients to make a tornado include:

• Moist, warm air at low elevations—readily available from the Gulf of Mexico.

• Cool, dry air at higher elevations—readily available from Canada.

• A strong southerly wind at the surface, plus a strong wind aloft blowing above it from a different direction, usually westerly or southwesterly, which creates wind shear and gets the air spinning.

• Some lifting mechanism, such as a cold front or storm line, which drives the warm air upward.

Every year -- particularly in March through June -- these ingredients come together in Tornado Alley to produce multi-day tornado outbreaks that are stunning in their power and devastation.

Thanks to Doppler radar, field studies and computer modeling during the past few decades, meteorologists have made much progress in understanding tornadoes. Yet as distinguished tornado expert Robert Davies-Jones explains in his 1995 article, "the storms that spawn twisters are now largely understood but mysteries still remain about how these violent vortices form."

The deadliest and most expensive natural disasters in U.S. history, however, were both hurricanes: the great Galveston hurricane of 1900, which killed some 6,000 to 12,000 people, and the notorious Hurricane Katrina in 2005, which caused an estimated $149 billion in damages. Scientific American covered both great storms. Isaac M. Cline, chief meteorologist at the Galveston, Tex., office of the U.S. Weather Bureau (later the National Weather Service) during the Galveston hurricane, wrote "Special Report on the Galveston Hurricane of September 8, 1900," in the October Supplement. Cline's pregnant wife perished in the storm, and he himself narrowly escaped drowning in the devasting storm surge.

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