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The Straight Dope


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From years of watching TV weather I know that our weather (in particular the jet stream) moves from west to east. Yet, as the sunrise and sunset show, the earth rotates in that same direction. Does that mean that the atmosphere is rotating around the planet faster than the planet itself is spinning? Shouldn't it be just the opposite? --Charles Barksdale, Baltimore

It would be if the world were run by newspaper Q&A columnists. You try explaining convection currents and the Coriolis effect in 600 words or less. Fact is, though, there's no reason to expect the winds and weather to move slower than the planet. Ignoring all other considerations, friction alone would keep the atmosphere spinning at the same rate as the earth beneath it. So why do the jet stream and weather in general move faster than the earth? First the simplified explanation. Convection currents in the temperate latitudes tend to push the surface winds north. The rotation of the earth tends to push anything northbound toward the east. Why? Because things near the equator are moving east faster than things near the pole. At the equator you're rocketing east at 1,000 miles an hour due to the earth's rotation. At the North Pole everything is pretty much stationary. As you fly north from equator to pole, therefore, you find you're moving east faster than the earth beneath you. Same with the winds--they're deflected east. This is called the Coriolis effect.

Now to confuse matters. The prevailing winds are out of the west only in the temperate latitudes. In the tropical and polar regions they're out of the east, also due to convection currents and the Coriolis effect. Hot air at the equator expands, rises, and (in the northern hemisphere) moves north. At 25 to 30 degrees north latitude it cools off, sinks, and heads south again. Something similar happens in the arctic. Frigid polar air contracts, sinks, and heads south to about 60 degrees north, where it warms up, rises, and returns north. In both cases convection drives the surface winds south. The Coriolis effect pushes anything southbound toward the west, for reasons I will let you noodle out for yourself. So the prevailing winds are out of the east.

You now ask: why does convection in the temperate latitudes seem to work the reverse of polar and tropical convection? Because in our region of relatively moderate temperatures, atmospheric circulation is driven by the extremes to the north and south. As a result, everything is backward. At the southern boundary of our zone the air is driven down (not up) by tropical convection, then heads north (not south) to the northern boundary, where it's pushed up (not down) by polar convection.

You don't get it, I know. The English language is a pathetic vehicle for this sort of thing. Perhaps Slug's drawing will make matters a little clearer. If not, take heart: I could have dragged in conservation of angular momentum, but didn't. There are some things mere mortals just don't need to know.

Who were the "buffalo soldiers"? I recently bought a set of U.S. postal stamps featuring buffalo soldiers and depicting flag-carrying gunmen on horseback. One wears a cap of roughly Civil War vintage. My first thought was that these chaps were part of the campaign to eliminate the buffalo and the native peoples dependent on them, and that the post office was perhaps competing for a Least Politically Correct Stamp Award. My dad, however, suggested that "buffalo soldier" might have been a name given to black soldiers after the Civil War, which for some reason has the ring of truth. What's the story here? --Paul Baber, Oakland, California

Listen to Pops. The stamp, issued in April 1994, honors the first African Americans recruited into the peacetime army, where they served in segregated regiments. The black troopers were nicknamed "buffalo soldiers" by Indians, who thought their hair was similar to that of the buffalo. The buffalo soldiers served primarily in the American west and also saw action during the Spanish-American War and subsequent conflicts. They were often given inferior equipment and subjected to other indignities, but nonetheless served with distinction. Eighteen buffalo soldiers were awarded the Medal of Honor, and black regiments reportedly had the lowest desertion rates of all U.S. army units between 1867 and 1898. If Elvis rates a stamp, it's the least we can do for these guys.

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

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