A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Why does weather move west to east?

December 16, 1994

Dear Cecil:

From years of watching TV weathermen I know that our weather (in particular the jet stream) moves from west to east. Yet, as the sunrise and sunset indicate, 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?

Cecil replies:

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. The main thing to remember is that the weather moves from west to east only in the part of the world where you happen to be, namely the temperate latitudes. In both tropical and polar regions the prevailing winds move in the opposite direction, from east to west. Why? Convection currents and the Coriolis effect. Hot air at the equator expands, rises, and (in the northern hemisphere) moves north. By the time it gets to about 25-30 degrees north latitude it cools off, sinks, and heads south again. The Coriolis effect deflects the moving air to the right enroute — that is, toward the west — and the net effect is that the prevailing winds, better known as the trade winds, are out of the northeast.

Something similar happens in the arctic. Frigid polar air contracts, sinks, and heads south. The Coriolis effect deflects it to the right (west) and we wind up with the polar easterlies.

In between — that is, in the zone from 30 to 60 degrees latitude — we have us. In our region of relatively moderate temperatures, general circulation of the atmosphere is driven by the extremes to the north and south. As a result, everything is backwards (ain't it the truth?). At the southern boundary of our zone the air is driven down by tropical convection and thereafter heads north. As usual the Coriolis effect deflects it to the right, which in this case means to the east. The upshot: the prevailing winds are out of the west, just like on the TV weather maps.

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.

Return of the planet of the hyperactive alien schoolmarms

Dear Cecil:

The grammarian microchip in your incontrovertibly mighty brain was clearly malfunctioning when you wrote, "You try explaining convection currents and the Coriolis effect in 600 words or less." What you meant to say was "600 words or fewer." I suggest you pull a Newt here and blame this embarrassing solecism on snarky liberal/Commie McGoverniks in the Straight Dope proofreading division.

Cecil replies:

David, how long have you been reading this column? The proper response on coming across some apparent mistake is not, "Hah! Cecil messed up!" but rather, "How is it that I have been mistaken all these years?" I quote from the American Heritage Dictionary, third edition: "Less is sometimes used with plural nouns in the expressions no less than (as in No less than 30 of his colleagues signed the letter) and or less (as in Give your reasons in 25 words or less)."

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