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Born At The Equator
Fri Dec 26, 2003
Listen in RealAudio 
A long-standing weather rule held is that tropical cyclones never form within 180 miles of the
Equator. In late-December 2001, one storm changed that thought. Hi, I'm Bryan Yeaton, and this
is The Weather Notebook.
Meteorologists felt that the belt ten degrees either side of the Equator was cyclone-free
because the Coriolis effect was too weak to spin a budding tropical depression enough to form
or sustain an organized storm rotation.
The Coriolis effect, produced by the Earth's rotation, is non-existent directly on the Equator
and increases in magnitude as one travels toward the Poles. It's the force that gives
mid-latitude cyclones their spin and veers all large-scale motion toward the right in the
Northern Hemisphere. It does not affect smaller scale flows such as dust devils, nor toilet
drains. That's a myth!
When pockets of intense tropical thunderstorms form over warm equatorial ocean waters, they
don't develop the characteristic counterclockwise spin of tropical storms unless they move to
higher latitudes, where the Coriolis force could get them spinning, and enable them to
intensify.
But, December 27, 2001, in the South China Sea, 1.5 degrees, about 100 miles north of the
Equator, tropical forecasters announced the appearance of Typhoon Vamei, and long-held beliefs
changed.
Later analysis revealed that a weak, quasi-stagnant disturbance off Borneo interacted with a
strong, cold surge off Asia that set up a background rotation when it hit the island. When
surge met disturbance, spin happened, and a typhoon rapidly emerged that had winds howling in
both hemispheres.
This confluence of events has been estimated to occur only once every 100 to 400
years.
Thanks to our contributing writer, meteorologist Keith Heidorn. Our show is funded by Subaru
and The National Science Foundation.
Today's Links
Typhoon Vamei
http://www.weather.nps.navy.mil/cpchang/papers/vamei/vamei.htm
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