Apparent wind is the wind force experienced by an object moving through the air. It is different than true wind, or the wind experienced by a stationary object. The concept of apparent wind is important in sailing, where it helps determine the conditions a craft can effectively travel in. In airplane flight, apparent wind has a similar effect. Airplanes usually take off and land into the wind in order to reduce the ground speed at which the aircraft takes off or lands.
Apparent wind is calculated as the vector sum of the true wind and the headwind component that the moving object would experience in still air. All values are vectors, which have both magnitude, or length, and direction. Vector can therefore be conceptualized as straight lines with start points and end points. A vector sum is calculated by placing the end point of one vector at the start point of the other; the sum is a vector from the start point of the first vector to the endpoint of the other.
Headwind itself is the inverse of the velocity of a moving object, meaning the start point and end point of the vector are reversed, with the same magnitude. Therefore, the apparent wind can also be calculated as the velocity of a moving object subtracted from the true wind velocity. For nautical purposes, the direction of velocity is measured in degrees, and the magnitude in knots, or nautical miles per hour. Traveling directly upwind gives an angle of 0 degrees, and directly downwind gives an angle of 180 degrees. A knot is approximately approximately 1.151 miles per hour (1.852 km per hour).
Apparent wind can also be used to measure true wind from a moving object, as the true wind can be calculated given the measurement of the wind as experienced on the moving object and the measurement of the object's velocity. Most modern sailing vessels are equipped with an anemometer, which measures wind velocity and pressure, and a wind vane, which measures the direction of the wind. The apparent wind is measured by these instruments, with wind direction measured in degrees relative to the direction in which the vessel is traveling.
An object moving against the wind experiences increased drag and decreased efficiency, resulting in decreased acceleration. Some vessels can travel faster than the true wind, especially if they are of light construction, causing less drag when traveling against the wind. Windsurfers, some mutlihulls, ice-sailors, and land-sailors are some of the crafts able to travel faster than true wind.