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Why should I care about the barometric pressure? Just about every TV weather report I've ever seen mentions it, along with whether it's rising or falling, as if this is supposed to be meaningful. Well, not to me it isn't. What's the story? Is this just a useless piece of erudition the weather creatures throw in to make their forecasts sound scientific, or am I missing out on something? --Mary M.Q.C., Chicago

Could be, buttercup--ever try it with raspberry sauce on the beach at midnight? Definitely an experience. But I suppose that's not what you had in mind. Actually, it's not true that all TV weather forecasts mention barometric pressure. More often than not, in fact, they skip it, for just the reason you'd expect--most folks have only the vaguest idea what it means. Barometer readings are nonetheless an indispensable part of weather forecasting, because pressure differences are what make the winds blow. More to the point, areas of high and low pressure are the pivots about which regional wind systems rotate.

The winds around a low, known as cyclones, always spiral inward in a counterclockwise direction. Those around a high, which are known as anticyclones, spiral outward in a clockwise direction. Too complicated? Just remember this: if the wind's at your back, there's either a low on your left (two L words, got it?), a high on your right, or both. Cyclones and anticyclones, incidentally, can cover half a continent and are not to be confused with the much smaller and more vicious local wind systems known as tornados, to which the term cyclone is sometimes erroneously applied.

If you can guess where the highs and lows are going to be at a given time, you'll be able to predict the winds, and if you can do that, you'll have a pretty good idea what the weather is going to be like. (Of course, you'll also want to know the location of any warm or cold fronts, which often rotate around lows like spokes, but let's not complicate the issue.)

Many people tend to associate high pressure with fair weather and low pressure with storms. That's true up to a point, but it's just as important to know whether the pressure is rising or falling. If the barometer is at 30 inches or more and rising, you can be reasonably sure there are clear skies ahead, whereas if it's below 30 and falling, you can expect clouds and precipitation. On the other hand, if the barometer is steady, or if it's low and rising or high and falling, the weather is less predictable.

Why do highs and lows have the effect they do? It's all a matter of physics. The winds spiral in toward a low for the obvious reason that air tends to flow from areas of high pressure to areas of low pressure. (The spiraling motion is imparted by the rotation of the earth, a phenomenon known as the Coriolis effect.) When all the air arrives in the center of the low it has no place to go but up. The air cools off as it rises and the moisture in it condenses, forming clouds and possibly rain or snow. Ergo, lows mean storms.

The same principles apply to highs. The winds spiral outward from a high because air flows from high pressure to low. New air flows down from above to replace the air spiraling out. The down-flowing air warms up en route, causing any clouds to evaporate. Ergo, highs mean fair weather. Sounds simple, no? Believe me, it's a lot more complex than I make it out to be. But at least you'll be able to get through the weather forecast.

We were talking about the medieval theory of the four bodily humors and noticed that "sanguine," "choleric," and "melancholy" correspond roughly in meaning with the modern English "glad," "mad," and "sad." We were wishing we knew a rhyming word for "phlegmatic." Can you think of one? --Justin Quisitive, Arlington, Massachusetts

The medieval theory of the four bodily humors, eh? Well, I guess it beats talking about what's on sale at K mart. Given that "phlegmatic" means calm, sluggish, and unemotional, I vote for glad, mad, sad, and moss-clad. What do you want, Shakespeare?

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

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