Dear Tom,
What is the difference between statute and nautical miles?. Why are wind speeds used in meteorology and air travel measured in nautical miles instead of statute miles?
Linda Klemka-Walden
Dear Linda,
Non-mariners and aviators in this country are most familiar with the statute mile which extends 5,280 ft. By comparison, a nautical mile equals 6,076.11549 ft. To convert from statute to nautical miles, multiply the number of nautical miles by 1.150779 (this sentence as published has been corrected in this text).
Navigators prefer using nautical miles because of the simple relationship to the latitude/longitude system which identifies locations on this planet. One nautical mile equals 1/60th of 1 (degree) of Earth’s circumference.
The use by mariners of knots to express wind speeds began because of a device known as a log chip and line, used in the early days of sailing to measure the speed of a ship’s movement. Rope, knotted in 47 ft. 3 inch intervals and tied to a piece of wood, was allowed to unwind behind a ship once thrown overboard for exactly 28 seconds. 28 seconds are to one hour what 47 ft. 3 inches are to a nautical mile. As a result, by counting the number of knots which unravelled on the line behind the ship, sailors could assign a speed to the craft-i.e. a length of line with 5 knots on it meant the ship was moving at 5 knots (5 nautical m.p.h.).
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Tom Skilling is chief meteorologist at WGN-TV. His weather forecasts can be seen Monday through Friday on WGN News at noon and 9 p.m.
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