Knot (unit) fluids (tidal streams, river currents and wind speeds) are also measured in knots. Thus, speed over the ground (SOG) (ground speed (GS) in aircraft) and rate of progress towards a distant point ("velocity made good", VMG) are also given in knots. Until the mid-19th century, vessel speed at sea was measured using a chip log. This consisted of a wooden panel, attached by line to a reel, and weighted on one edge to float perpendicularly to the water surface and thus present substantial resistance to the water moving around it. The chip log was cast over the stern of the
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