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The Straight Dope


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I have never heard of a tornado occurring outside North America. Is this weather phenomenon unique to North America? Is this because only North America has mobile-home parks, which attract tornadoes? --Dean, Dallas

Silly boy. Tornadoes occur throughout the world. I have reports here of tornadoes from Moscow, Peshawar (Pakistan), even Vienna. But they do occur most frequently in North America, and in particular in the central plains of the U.S. Something like 700 tornadoes a year occur in the U.S.; Australia, with 200, is a distant second.

Roughly 90 percent of U.S. twisters strike in a 300-mile-wide corridor extending from west Texas to Canada. That's because conditions there are ideal for tornado formation. First you get warm, moist surface winds blowing up from the Gulf of Mexico, while cool high-altitude winds blow over the tops of the Rockies. This situation is inherently unstable because cool air wants to sink while warm air wants to rise. However, for reasons we need not delve into here, the mountain air causes a temperature inversion, which prevents the warm surface air from rising. It's like clamping the lid on a pressure cooker. The surface weather systems build up a big head of steam until they break through the inversion, whereupon they shoot up to towering heights. This sets in motion the violent up- and downdrafts that lead, by circumstances still imperfectly understood, to the formation of more tornadoes than anywhere else in the world.

Last summer when I was atop Mount Quandary near Denver, I asked my climbing companions exactly what caused the sudden absence of growth referred to as the "tree line." My friends, local granola-head types, tried to explain that the lack of oxygen caused the tree line. I'm not a scientist, but I did at least remember from high school that trees depend on carbon dioxide, not oxygen. We then theorized at length without coming to any conclusions. One friend said maybe it was the temperature or the sparse soil at high altitudes. Another thought the cause somehow had to be the angle of the sun. I think the answer has to do with the lower pressure at the higher altitudes interfering with photosynthesis. I've heard that the tree line is lower the further away a mountain is from the equator. If my pressure/photosynthesis theory is correct, is the atmosphere thinner as you move away from the equator, resulting in lower pressure at lower levels and, thus, lower tree lines? --Don Moore, Miami

You started off promisingly by remembering about trees and carbon dioxide, Don, but things went to hell thereafter. Your friends' theories are closer to the mark than yours. The main factor in determining the tree line is the temperature--trees can't survive if it stays cold more than a certain percentage of the year. Since cold increases with both (1) distance from the equator and (2) elevation, the tree line, also known as the timberline, is lower the closer you get to the poles. Soil and drainage are also factors. The cold temps near the polar regions are a function of the angle of the sun, so there's something to be said for that explanation. There's also something to be said about your pressure/photosynthesis idea, but I'm feeling uncharacteristically agreeable this morning and won't say it.

What is a Hoosier? --Jeremy Shea, Madison, Wisconsin

Somebody who lives in Indiana, naturally. Probably you knew this. Probably the question that really bugs you is where the term Hoosier originated. We don't know exactly, because the word was commonly used to describe an Indianan as early as 1826, long before the etymologists had had a chance to unpack their notepads. The best guess comes from Jacob Dunn, who in Indiana and the Indianans (1919) put forth the proposition that it came from the Cumberland dialect word hoozer, meaning anything unusually large--presumably in the sense of "you unusually large galoot," since early Indianans were regarded as burly rustics. Things are different today. Now they're 98-pound rustics.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.

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