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Trade Winds
Fri Mar 14, 2003
Listen in RealAudio 
Hi, I'm Bryan Yeaton for The Weather Notebook.
The term "trade wind" conjures up an image of old clipper ships propelled briskly past
spice-laden islands by steady tropical breezes -- and for once the fantasy has some truth to
it, at least as some meteorological truth. The trades are in fact remarkably constant
tropical winds associated with relatively tranquil weather -- perfect for speeding clipper
ships across the sea. Provided that your ship is heading west. For trade winds, by
definition, always blow from east to west.
Equatorial heat is the prime mover behind trade winds. At the equator, hot air rises through
the troposphere and then travels poleward until it reaches about 30o North or South latitude.
By that distance, the air has cooled enough to descend once more to the surface, where it is
drawn back to the equator to replace the rising hot air. If the earth were stationary, these
vast circulating cells would blow from opposite directions in the two hemispheres -- but the
rotation of the earth deflects the air currents, creating northeasterly trades in the northern
tropics and southeasterly trades in the southern hemisphere.
The trades not only make for smooth westbound sailing, but they provide ideal tropical
vacation weather, with low humidity and balmy temperatures. For a taste of this meteorological
paradise, head for Hawaii, but make sure you go in summer when the trades prevail about 90% of
the time -- nearly twice their winter rate. On second thought, better put that trip on hold
until the current El Niņo dissipates. One of the first signs of an outbreak of El
Niņo is a pronounced weakening of the trades.
David Laskin contributed today's story. Our program is produced by the Mount Washington
Observatory. We receive generous support from Subaru of America, and the National Science
Foundation.
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