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 gradient variation in wind speed with height can be defined relative to wind measured at a reference height of 10 meters as: where: The Hellmann exponent depends upon the coastal location and the shape of the terrain on the ground, and the stability of the air. Examples of values of the Hellmann exponent are given in the table below: Source: "Renewable energy: technology, economics, and environment" by Martin Kaltschmitt, Wolfgang Streicher, Andreas Wiese, (Springer, 2007, , ), page 55 In gliding, wind gradient affects the takeoff and landing phases of flight of a glider. Wind gradient can have a noticeable effect