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 Measurement

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meodingu
Kig 'Yar
Kig 'Yar


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Number of posts : 141
Age : 31
Registration date : 2010-10-13

PostSubject: Measurement   Tue Oct 26, 2010 6:47 am

Measurement

A windmill style of anemometer

An occluded mesocyclone tornado (Oklahoma, May 1999)

Wind direction is reported by the direction from which it originates. For example, a northerly wind blows from the north to the south.[6] Weather vanes pivot to indicate the direction of the wind.[7] At airports, windsocks are primarily used to indicate wind direction, but can also be used to estimate wind speed by its angle of hang.[8] Wind speed is measured by anemometers, most commonly using rotating cups or propellers. When a high measurement frequency is needed (such as in research applications), wind can be measured by the propagation speed of ultrasound signals or by the effect of ventilation on the resistance of a heated wire.[9] Another type of anemometer uses pitot tubes that take advantage of the pressure differential between an inner tube and an outer tube that is exposed to the wind to determine the dynamic pressure, which is then used to compute the wind speed.[10]

Sustained wind speeds are reported globally at a 10 meters (33 ft) height and are averaged over a 10 minute time frame. The United States reports winds over a 1 minute average,[11] while India typically reports winds over a 3 minute average.[12] Knowing the wind sampling average is important, as the value of a one-minute sustained wind is typically 14 percent greater than a ten-minute sustained wind.[13] A short burst of high speed wind is termed a wind gust, one technical definition of a wind gust is: the maxima that exceed the lowest wind speed measured during a ten minute time interval by 10 knots (19 km/h). A squall is a doubling of the wind speed above a certain threshold, which lasts for a minute or more.

To determine winds aloft, rawinsondes determine wind speed by GPS, radio navigation, or radar tracking of the probe.[14] Alternatively, movement of the parent weather balloon position can be tracked from the ground visually using theodolites.[15] Remote sensing techniques for wind include SODAR, Doppler LIDARs and RADARs, which can measure the Doppler shift of electromagnetic radiation scattered or reflected off suspended aerosols or molecules, and radiometers and radars can be used to measure the surface roughness of the ocean from space or airplanes. Ocean roughness can be used to estimate wind velocity close to the sea surface over oceans. Geostationary satellite imagery can be used to estimate the winds throughout the atmosphere based upon how far clouds move from one image to the next. Wind Engineering describes the study of the effects of the wind on the built environment, including buildings, bridges and other man-made objects.









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