Dear Umbra,

Why does the jet stream move from west to east?

Pittsburgh, Penn.

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Dearest Joe,

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To get a decent bagel?

Image: NASA.

There are actually two main global jet streams, the “polar” in the Northern Hemisphere and the “subtropical” in the Southern Hemisphere. The ridiculously simple answer to your question is that jet streams move west to east because the Earth rotates west to east in its orbit. (It also matters that the planet is hotter at the equator than the poles.) A longer explanation would involve Very Complicated Science. Rather than confuse everyone with an overdose of information on angular momentum, pressure gradients, and the Coriolis effect, I’m going to point you toward some resources with which you can do your own atmospheric science research project. From the basic to the moderately complex and beyond, you can find all the details you want on the web. The most comprehensible source would probably be a climate-science textbook, if you happen to be near a college bookstore.

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Even for those of us who are neither pilots nor atmospheric scientists, it is important to know a little about the jet stream. The jet streams are giant currents of air 30,000 feet in the atmosphere; as they travel, they bend and curve their course over mountains and oceans, and carry smaller pressure systems with them like so many logs in a river. The smaller pressure systems are responsible for our weather down below, so you may have the jet stream to thank for your snowy driveway, or that torrential downpour. The vital concept for us environmentally concerned folks is that the jet streams flow where they do because of the current temperature patterns of the Earth and its atmosphere. If and when the temperature shifts permanently, the jet streams will shift also, taking all the familiar weather with them.

The shifting of the jet streams is one of the nightmare scenarios of global warming. If the planet and especially the oceans warm sufficiently, the polar jet stream could migrate substantially southward, causing a radical drop in the average temperature in Northern Europe, among other severe environmental consequences. The weather everywhere will permanently change. Oceans may rise, eventually flooding the beach where President Bush and Vice President Dick Cheney have stuck their heads in the sand.