Apparent wind

From Freepedia

Apparent wind is the breeze felt by an observer. It is created both by the movement of the observer, and by the true wind. For a stationary observer, both apparent wind and true wind are the same.

Apparent wind is calculated by the sum of two vectors. The first vector is the true wind velocity. The second vector is the velocity of the observer. By adding these two vectors together, one can determine the apparent wind.

In sailing, and especially in sailboat racing, apparent wind is a vitally important factor. In a race, or when sailing in general, apparent wind determines the points of sail on which your sails can efficiently generate forward motion.

In fixed-wing aircraft, apparent wind determines the necessary speeds for takeoff and landing. Aircraft carriers generally steam directly upwind at speeds of up to 35 knots, in order to increase apparent wind and reduce the necessary takeoff velocity. Land-based airport traffic generally takes off and lands facing upwind, for the same reason.