Apparent wind

Apparent wind is the wind a moving vessel actually feels, the vector sum of the true wind and the wind made by the vessel’s own motion through the water. It differs from true wind in both speed and direction: steaming to windward gives a stronger, more head-on apparent wind; running before it gives a weaker, more astern one. Masthead anemometers read apparent wind, and the observer resolves true wind from it on a wind triangle using the ship’s course and speed. The distinction matters for sail trim, helicopter and crane work, and reducing observed wind to the true wind logged in synoptic reports.