The Weather Notebook: Understanding the Jet Stream

Understanding the Jet Stream
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The jet stream is well named for it is indeed a kind of stream. It's actually a stream of air that flows through the upper atmosphere, about 7 miles high. Jet stream speeds of about 75 mph in winter and 35 mph in summer are typical, though the jet has been clocked at 200 mph. Hi, I'm Dave Thurlow from the Mount Washington Observatory and this is The Weather Notebook.

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The jet stream is propelled by the contrast in temperature and air pressure between the equator and the poles. Since the equator receives the maximum in solar heating and the poles the minimum, a strong temperature difference generates a strong pressure difference- the classic formula for wind.

Knowing only this, you'd conclude that the jet stream, or rather jet streams - one for each hemisphere - would flow north and south to their respective poles. The reason they don't is the Coriolis Effect, the apparent deflection of free-moving objects - like air - relative to the rotation of the earth. The Coriolis Effect turns both the Northern and Southern hemisphere jets away from their poleward courses and makes them circulate the globe in the same direction - west to east.

If the jet streams ran straight like lines of latitude our weather would be extremely monotonous; but, for better or worse, the unevenness in temperature and the disruptive presence of land masses cause the jets to meander in giant dives and ascents. These continent-wide loops interact with smaller waves or "eddies" of secondary atmospheric circulations that whip up more localized disturbances.

Of course there's more to weather forecasting than understanding what the jet is up to - but if you want to put the big picture in focus, it's the essential first step.

Thanks to David Laskin of Seattle, today's contributing writer.