True wind

In marine meteorology and navigation, true wind represents the actual motion of air across the earth's surface relative to a fixed geographic position. Unlike apparent wind, which is felt aboard a moving vessel, true wind is independent of boat speed and heading. According to Wikipedia's article on points of sail, a sailing craft's point of sail is officially defined relative to this true wind direction over the water.

Key parameters include True Wind Speed (TWS), measured in knots (1 knot = 1.852 km/h) or meters per second (m/s), True Wind Direction (TWD), expressed in degrees from true North (0°–360°), and True Wind Angle (TWA), measured relative to the vessel's bow. To properly distinguish homonyms and technical nuances, TWD must not be confused with magnetic wind direction, nor should TWS be mistaken for boat-generated wind.

Onboard marine electronics calculate true wind vectorially by subtracting the boat's speed through water (STW) or speed over ground (SOG) and heading from the apparent wind vector. Accurate true wind data is crucial when evaluating marine weather forecast sites and understanding true and apparent wind differences for strategic routing and sail selection.