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Miscellany 51: Apparent Wind Speed and True Wind Speed (7 December 2009, revised 9 December 2009, minor edits 9 February 2010.)
The “apparent wind” is the wind that “appears” to you, whether you are moving (relative to the ground) or standing still (not moving relative to the ground). The “true wind” is the apparent wind while you are standing still. Wind speed is measured by an anemometer, and wind direction (the angle of the wind to the boat) is measured by a wind-direction indicator (e.g., burgee or tell-tale). (If the boat is not moving, these measurements are true wind speed and true wind angle; if the boat is moving, these measurements are apparent wind speed and apparent wind angle.) This note derives a formula for calculating apparent wind speed.