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Subject: D10) Why do hurricane force winds start at
64 knots ?
Contributed by Neal Dorst
In 1805-06, Commander Francis Beaufort RN (later Admiral Sir Francis
Beaufort) devised a descriptive wind scale in an effort to standardize
wind reports in ship's logs. His scale divided wind speeds into
14 Forces (soon after pared down to thirteen) with each
Force assigned a number, a common name, and a description of the
effects such a wind would have on a sailing ship. And since the
worst storm an Atlantic sailor was likely to run into was a hurricane,
that name was applied to the top Force on the scale.
During the 19th Century, with the manufacture of accurate
anemometers, actual numerical values were assigned to each Force
level, but it wasn't until 1926 (with revisions in 1939 and 1946)
that the International Meteorological Committee (predecessor of
the WMO) adopted a universal scale of wind speed values.
Since the scale was originally nautical in nature, and that most
wind reports at that time were in nautical miles per hour
(or converted to them), these numerical values were given in knots.
It was a progressive scale with the range of speed for Forces
increasing as you go higher. Thus Force 1 is only 3 knots in range
(1 kt - 3 kt), while the Force 11 is eight knots (56 kt - 63 kt) in range.
So Force 12 [Hurricane] starts out at 64 knots (74 mph, 33 m/s).
There is nothing magical in this number, and since hurricane
force winds are a rare experience, chances are the committee
which decided on this number didn't do so because of any real
observations during a hurricane. Indeed, the Smeaton-Rouse
wind scale in 1759 pegged hurricane force at 70 knots
(80 mph, 36 m/s). Just the same, when a tropical
cyclone has maximum winds of approximately these speeds we
do see the mature structure (eye, eyewall, spiral rainbands)
begin to form, so there is some utility with setting hurricane
force in this neighborhood.
References
- Hamblyn, Richard "The Invention of Clouds : How an
Amateur Meteorologist Forged the Language of the Skies",
(2001) Farrar, Straus, and Giroux New York, NY
- DeBlieu, Jan "Wind : How the Flow of Air Has Shaped
Life, Myth, and the Land" (1998) Houghton Mifflin Co. New York, NY
Last updated August 13, 2004
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