Apparent wind in knots and degrees. Note that a number of additional factors come into play when converting the measurements from the masthead anemometer into the true wind if a high degree of accuracy is required, including the following: In the presence of a current, the true wind is considered to be that measured on the craft drifting with the water over the bottom, and wind with respect to the sea bed as the "ground" or "geographical wind". The apparent wind on board (a boat) is often quoted as a speed measured by a masthead transducer containing an anemometer and wind vane
Wind power the volume of air passing through "A" (which is considered perpendicular to the direction of the wind); "Avtρ" is therefore the mass "m" passing through "A". Note that ½ "ρv" is the kinetic energy of the moving air per unit volume. Power is energy per unit time, so the wind power incident on "A" (e.g. equal to the rotor area of a wind turbine) is: Wind power in an open air stream is thus "proportional" to the "third power" of the wind speed; the available power increases eightfold when the wind speed doubles. Wind turbines for grid electric power therefore