The instrument used for measuring wind is called anemometer, a word derived from the Greek anemous, meaning wind. Anemometers record wind speed in either meters per second or knots (nautical miles per hour). Today, fixed velocity levels are categorized in accordance with the Beaufort Scale.
The simplest measuring system is based on a rotating fan wheel. The stronger the wind, the faster the fan blades spin. However, wind speed can also be measured using dynamic pressure in a small tube. The faster the air stream that enters the tube, the more pressure it exerts on a gas or liquid. The same principle underlies the Pitot tube (named after its inventor, the French engineer Henri Pitot, 1695-1771), which is used today for measuring the speed of aircraft. Other measuring devices use ultrasound or the cooling effect of the wind on a heated wire to measure the speed of air currents. According to an internationally agreed standard, wind speed is measured at a height of 10 m, because speeds on the ground can be subject to local variations that lead to incorrect measurements.