Excerpt: Storm Chaser Reed Timmer's 'Into the Storm'

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Usually the rising air in a cloud quickly cools as it ascends, and the cloud stops moving up. But not dur-ing the formation of an F5. The rising air—the updraft—won't die. Condensation continues, and the traces of ascending mist accumulate, not unlike the way a snowball gathers more snow as it rolls downhill. But in the case of an F5, the "snowball" is climbing toward the heavens. On a humid spring day, folks in Tornado Alley can turn their backs on the sky for just minutes, to mow a lawn or wash a car. When they look up again, what was a cloud-free sky before has become blemished—more like dominated—by a lone, sunlit, bright white, ominous cloud.

The crisp cloud in my mind rapidly grows from a height of hundreds to thousands to tens of thousands of feet, blasting through the troposphere and into the stratosphere until it's thirteen miles tall. This is a cu-mulonimbus cloud, and in its most radical, towering form people say that it looks like the mushroom cloud that's associated with a nuclear explosion. Considering what an F5 tornado can do to both property and people, the metaphor isn't too far-fetched.

According to the Fujita Scale upon which the "F5" classification is based, such a tornado can flatten homes, turn cars into airborne missiles, and debark trees. The Fujita Scale is a widely accepted "damage scale" for categorizing tornadoes that's based on the havoc they wreak, and "F5" is used only to identify the most destructive rotating winds—those that spin anywhere from an estimated 261 to 318 miles per hour. Created by University of Chicago meteorologist Tetsuya "Ted" Fujita in 1971, the scale's other five catego-ries also use damage characteristics and estimated speeds to classify every other tornado: F0 (under 73 miles per hour); F1 (73 to 112 miles per hour); F2 (113 to 157 miles per hour); F3 (158 to 206 miles per hour); and F4 (207 to 260 miles per hour). In 2007, the "F-scale" was supplanted by a slightly modified "En-hanced Fujita Scale," or "EF-scale" (for consistency I've stuck with the F-scale, which was in use for most of my early days of storm chasing, throughout this book). Whichever scale you use, to suggest "F5" is to suggest almost unfathomable power.

Sure enough, near the base of this imaginary and towering cumulonimbus cloud, forces are at work. Winds howl through the cloud at varying elevations, directions, and speeds. One gale from the southwest might blow at fifty miles per hour at an elevation of six thousand feet; another blows at sixty miles per hour, from the west and at ten thousand feet; a third gust maintains twenty miles per hour near the earth's surface. This phenomenon is called wind shear. The conflicting and contrasting winds push and pull the air inside the cloud until, finally, that air moves in a uniform, circular current. Ultimately the air inside the cloud begins to rotate on a horizontal axis, much the same way that laundry rotates inside a clothes dryer.

The turbulence, however, has only just begun. The force of the updraft inside the cumulonimbus cloud acts like a crane, yanking and tilting the cylinder of rotating air inside the cloud as if it were a pipe that had to ultimately stand on one end. By the time the updraft inside the cloud is finished repositioning this cylinder of air, it's now spinning vertically, like a barber pole. Then the mesocyclone comes alive. I can picture it now—although anyone who is in the vicinity of such a cloud can't miss it.

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