Dear Tom,

I pay close attention to weather observations from the National Weather Service and other sources, but I have a question: Why is a wind speed of 11 miles per hour never reported anywhere?

Fred Bromburger, Berwyn

Dear Fred,

our question comes up occasionally. It may seem strange that a wind speed of 11 mph has never been reported. The explanation is that the National Weather Service measures wind speeds in knots — nautical miles per hour. But in the United States we use statute miles for measures of distance and stature miles per hour for measures of speed. One knot is 1.1508 statute miles per hour. Applying this conversion factor yields the following: 8 knots converts to 9 mph and 9 knots to 10 mph. However, 10 knots converts to 12 mph, and so 11 mph is skipped and does not appear in the conversion table.