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By Root 322 0

by fire vortexes. You can see vortexes in every campfire, in every little brook, in the water exiting a bathtub. Vortex motions, usually caused on Earth by pressure differentials in the atmosphere, are critical to a surprising number of human technologies, some economically important, others amusing but trivial. For example, airplane flight depends largely on vortex motion. Birds use tiny tornadoes to turn sharply in the air. The singing sound of high-tension wires is caused by vortexes. So is the ubiquitous slice of an amateur's golf ball, and, as we shall see when we get to Bernoulli's principle, even the annoying habit of shower curtains to billow inward and stick clammily to the person showering. A smoke ring is an example of a ring vortex. Some volcanoes blow smoke rings; so do steam locomotives. In nature, vortexes can have lifetimes ranging from a few seconds to several days.

In water, vortexes are called whirlpools, in which the flow is downward, and kolks, in which it is upward. Perhaps the most famous vortexes in history were Homer's Charybdis, off the Ca-labrian coast, and the Maelstrom, off Norway.

In air, the most common vortexes are whirlwinds and dust devils, which occur almost everywhere in the atmosphere, almost always accompanied by some degree of wind shear, or rapid interchange of air between layers; whole academic careers have been built on these boundary-layer studies. A typical whirlwind is the Australian cockeyed bob, which picks up leaves, light twigs, and dust as it goes; the vortex winds we called die duiwel gee om, "the devil cares," near where I was born, were strong enough to carry tumbleweeds, some of them as large as hippopotamuses. Winds that build from the bottom up, like these whirlwinds, are sometimes called willy-willys; their more deadly cousins, tornadoes, are made from the top down, and of course are far more ferocious.

Tornado comes from the Spanish word for thunderstorm, tronada, which in turn comes from the Latin for turn, tornare, which is what vortexes do. And turn tornadoes do, spinning tightly with awesome speed. This is the most violent of all winds. Just how violent is still unknown, because tornadoes routinely destroy even the most robust of measuring devices, even supposing one could be placed in a storm's unpredictable path. But estimates have placed vortex winds at somewhere around 290 miles an hour, much faster than a Category 5 hurricane like Camille, and it is possible that occasional tornado winds might even exceed 480 miles an hour. The highest wind ever measured within a tornado was near Red Rock, Oklahoma, in April 1991, when a twister was clocked at 286 miles an hour.

The story is similar for barometric pressure. Standard barometers can't cope with the rapid changes in pressure caused by tornadoes, but pressure drops of 100 millibars are not uncommon, and drops of 200 not unknown. Because such drops occur in mere seconds, the normal pressure inside a building simply doesn't have time to adjust before the roof is blown off and walls are blown outward. The energy within a single tornado is not much less than the 20-kiloton bomb dropped on Hiroshima.29

I've had three near-encounters with tornadoes, and I've seen the results of others. The first was just a few years after I had nearly been blown out to sea in a Cape southeaster. My family had moved to Johannesburg, a notorious locus of thunderstorms and massive hailstorms. One day we were in the family's aging sedan on our way from somewhere to somewhere else when the sky suddenly turned black, and then a violent yellow, and a deep rumble scraped across the city and across our nerves. My father brought the car to a stop and we saw the twisting funnel of a tornado touch down, perhaps a mile away, and then it was gone. Afterward he took me to see where it had been, a furrow of destruction two hundred yards wide carved through the edge of the city. Astonishingly, in the center of the path, a house still stood. Its roof was gone entirely, even the rafters, but nothing indoors was disturbed. Nearby was the remnant