Apparent wind that measures wind speed in knots and wind direction in degrees relative to the heading of the boat. Modern instrumentation can calculate the true wind velocity when the apparent wind and boat speed and direction are input. In sailboat racing, and especially in speed sailing, apparent wind is a vital factor, when determining the points of sail a sailboat can effectively travel in. A vessel traveling at increasing speed relative to the "prevailing wind" will encounter the wind driving the sail at a decreasing angle and increasing velocity. Eventually, the increased drag and diminished degree of efficiency of a sail
Wind profile power law of wind speed can provide different power densities for the same average wind speed. Wind profile power law The wind profile power law is a relationship between the wind speeds at one height, and those at another. The wind profile power law relationship is : where formula_2 is the wind speed (in metres per second) at height formula_3 (in metres), and formula_4 is the known wind speed at a reference height formula_5. The exponent (formula_6) is an empirically derived coefficient that varies dependent upon the stability of the atmosphere. For neutral stability conditions, formula_6 is approximately 1/7, or 0.143. In