differing temperature, elevation or barometric pressure, a correction is required to obtain the actual wind speed. Approximately 1.5% (1.6% above 6,000 feet) should be added to the velocity recorded by a tube anemometer for each 1000 ft (5% for each kilometer) above sea-level. Effect of icing At airports, it is essential to have accurate wind data under all conditions, including freezing precipitation. Anemometry is also required in monitoring and controlling the operation of wind turbines, which in cold environments are prone to in-cloud icing. Icing alters the aerodynamics of an anemometer and may entirely block it from operating. Therefore, anemometers used in these applications must be internally heated. Both cup anemometers and sonic anemometers are presently available with heated versions. Instrument location In order for wind speeds to be comparable from location to location, the effect of the terrain needs to be considered, especially in regard to height. Other considerations are the presence of trees, and both natural canyons and artificial canyons (urban buildings). The standard anemometer height in open rural terrain is 10 meters. See also Air flow meter Anemoi, for the ancient origin of the name of this technology Anemoscope, ancient device for measuring or predicting wind direction or weather Automated airport weather station Night of the Big Wind Particle image velocimetry Savonius wind turbine Wind power forecasting Wind run Windsock, a simple high-visibility indicator of approximate wind speed and direction Notes References Meteorological Instruments, W.E. Knowles Middleton and Athelstan F. Spilhaus, Third Edition revised, University of Toronto Press, Toronto, 1953 Invention of the Meteorological Instruments, W. E. Knowles Middleton, The Johns Hopkins Press, Baltimore, 1969 External links Description of the development and the construction of an ultrasonic anemometer Animation Showing Sonic Principle of Operation (Time of Flight Theory) – Gill Instruments Collection of historical anemometer Principle of Operation: Acoustic Resonance measurement – FT Technologies Thermopedia, "Anemometers (laser doppler)" Thermopedia, "Anemometers (pulsed thermal)" Thermopedia, "Anemometers (vane)" The Rotorvane Anemometer. Measuring both wind speed and direction using a tagged three-cup sensor Italian inventions Measuring instruments Meteorological instrumentation and equipment Navigational equipment Wind power 15th-century inventions