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 power in the United Kingdom and wind regimes, are smoothed, rather than intermittent. As the distance between sites increases, the correlation between wind speeds measured at those sites, decreases. The combined output of multiple wind turbines widely distributed across the UK gives a normal distribution of power, with a standard deviation which is about 0.76 of the average power. A Scottish government spokesman has said electricity generated by renewables accounted for 27% of Scotland's electricity use. On the night of 5–6 April 2011, the wind in Scotland was high, it was raining heavily, which also created more hydroelectricity than normal. The grid became overloaded preventing