Knots

The UK has seen metric units of measurement become the norm for most things, for example fruit and vegetables are now sold by the kilogram, rather than pound. This is also true in the metrological world, with temperature being read in Celsius rather than Fahrenheit.

Wind however is still measured in the imperial unit of knots. A knot is a nautical mile per hour, which is equal to 1.151mph or 1.85km/h.

The term knot came from mariners who used a “chip log” to measure the ships speed at sea. The chip log a “wooden panel” was thrown over the stern of the moving vessel, with a line attached to it. Knots were tied into the line at a distance of 47 feet and 3 inches and passed through the through the sailors fingers, the number of knots which passed through his fingers in a 30 second period determined the speed. 

Knots are still used today because the standard charts used for aviation and maritime purposes are in Mercator projection and so the scale varies with latitude. One nautical mile is equivalent to one minute of latitude, and so the speed in knots can be easily determined.