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Why you will never hear of a wind speed of 11 mph in a weather report

Dear Tom,
I have never heard a wind speed of 11 mph in a weather report, and I have listened for years.
Bill Walkor, Chicago
Dear Bill,
You are correct to observe that a wind speed of 11 mph has never been announced. This may seem strange, but here’s the explanation. The National Weather Service measures wind speeds in knots (nautical miles per hour) in accordance with international practice. However, in the United States we use statute miles for measures of distance and statute miles per hour for speed. So we measure wind speeds in knots, then convert that value to miles per hour. One knot is 1.1508 statute miles per hour; 8 knots converts to 9 mph and 9 knots to 10 mph. However, 10 knots converts to 12 mph, and so 11 mph does not appear in the conversion table.

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