When you think about it, wind is a strange phenomenon.
In its simplest form, wind occurs when uneven heating of the surface of the Earth causes higher pressure in some places than in others, and the air flows from highs to lows. But it's considerably more complex (and interesting) than that, because as surface-dwellers we often forget that there's a third dimension -- and that air can move vertically as well as horizontally.
I got to thinking of this because I've been reading Eric Pinder's fascinating, often lyrical, book Tying Down the Wind: Adventures in the Worst Weather on Earth. Pinder is a meteorologist who was stationed as a weather observer on Mount Washington, New Hampshire, which one in every three days clocks hurricane-force winds (greater than 119 kilometers per hour) and is the spot that holds second place for the highest anemometer-clocked wind speed ever recorded on the Earth's surface (an almost unimaginable 372 kilometers per hour; the only higher one was on Barrow Island, Australia, which on April 10, 1996, during Cyclone Olivia, hit 407 kilometers per hour).
The fact that air moves vertically, of course, is why air moves horizontally. When the Sun heats a patch of ground, the air above it warms and becomes less dense, causing it to rise. This creates an area of low pressure, and air moves in from the side to replace the air moving upward. This process, writ large, is what causes hurricanes; the heat source is the ocean, and the convection caused by that tremendous reservoir of heat energy not only generates wind, but when the water-vapor-laden air rises high enough, it undergoes adiabatic cooling, triggering condensation, cloud formation -- and torrential rain.
The process can go the other direction, though. A weather phenomenon that has long fascinated me is the convective microburst, something that most often happens in hot, dry climates in midsummer, like the American Midwest. The process goes something like this. Rising air triggers cloud formation, and ultimately rain clouds. When the droplets of water become heavy enough that the downward force of gravity exceeds the upward force of the air updrafts, they fall, but they drop into the layer of warm, dry air near the surface, so they evaporate on the way down, often not making it to the ground as rain. Evaporation cools the air that surrounds them, making it denser -- and if the process happens fast enough, it creates a blob of air so much denser than the air surrounding it that it literally falls out of the sky, hits the ground, and explodes outward. Windspeeds can go from nothing to 100 kilometers per hour in a matter of fifteen seconds. Then -- a couple of minutes later -- it's all over, the dust (and any airborne objects) settle back to Earth, and everyone in the vicinity staggers around trying to figure out what the hell just happened.
Microbursts aren't the only weird weather phenomenon having to do with density flow. Have you heard of katabatic winds? If you haven't, it's probably because you live in an area where they don't happen, because they're really dramatic where they do. Katabatic winds (from the Greek κατάβασις, "falling down") occurs when you have significant chilling of a layer of air aloft -- on top of a mountain, for example, or (even better) over an ice sheet. This raises the density of the air mass, creating a huge difference in gravitational potential energy from high to low. The superchilled air pours downward, funneling through any gaps in the terrain; the effect is accentuated when there's a low pressure center nearby. The katabatic winds off Antarctica (nicknamed "Herbies," for no reason I could find) and the ones off Greenland (known by the Inuit name piteraq) can be unpredictable, fast, and frigid, often driving layers of snow horizontally and creating sudden whiteout conditions.
Then there's the foehn (or föhn) wind, created when onshore air flow is pushed up against a mountain range. This occurs in the southern Alps, central Washington and Oregon, parts of Greece and Turkey, and south-central China. On the windward side of the mountains, the air rises and cools; this causes condensation and higher rainfall. But when the air piles up and gets pushed over the mountain passes, it warms for two reasons -- the pressure increases as it goes downhill on the other side, and the condensation of water vapor releases heat energy. The result is a warm, dry wind that pours downhill on the leeward side of the mountains -- the source of the "Chinook winds" that desiccate the northwestern United States east of the Cascades.
Interestingly, foehn winds are associated with physiological problems -- headaches, sinus problems, and mood swings. It's documented that prescriptions for anxiolytic medications go up when the foehn is blowing; and a study at the Ludwig Maximilians Universität München found that suicide and accident rates both go up by about ten percent during periods when there's a strong foehn, and no one knows why exactly.
In any case, there are a few interesting tidbits about a phenomenon we usually don't think about unless we're in the path of a hurricane or tornado. Something to think about next time your face is brushed by a warm breeze. We live at the bottom of a layer of moving fluid, driven by invisible forces that usually are benign. Only occasionally do we see how powerful that fluid can be -- preferably, from a safe distance.
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