Wind speed
From Wikipedia, the free encyclopedia
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calm (0–2 kn) |
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3–7 kn |
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8–12 kn |
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13–17 kn |
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18–22 kn |
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23–27 kn |
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28–32 kn |
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33–37 kn |
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38–42 kn |
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43–47 kn |
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48–52 kn |
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53–57 kn |
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58–62 kn |
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63–67 kn |
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98–102 kn |
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102–107 kn |
Wind speed is the speed of wind, the movement of air or other gases in an atmosphere. It is a scalar quantity, the magnitude of the vector of motion.
Wind speed has always meant the movement of air in an outside environment, but the speed of air movement inside is important in many areas, including weather forecasting, aircraft and maritime operations, building and civil engineering. High wind speeds can cause unpleasant side effects, and strong winds often have special names, including gales, hurricanes, and typhoons. See the Beaufort scale.
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[edit] Measurement
The simplest way to estimate wind speed is from observed phenomena. This is the basis of the Beaufort scale, where zero can be recognized by vertically rising smoke. As this method is only approximate, each value on the scale represents a range of speeds; for example three on the scale represents wind speeds between seven and ten knots.
For hundreds of years, the anemometer was the most accurate method of measuring wind speeds close to the ground. The simplest anemometers are based on a rotating vane, but most professional measurements are now made with a heated wire anemometer. The heated wire anemometer consists of a bare metal wire exposed to the wind which is heated by passing electrical current through it, as the wire becomes hotter its resistance increases and by measuring the temperature of the wind, the current flowing into the wire and the wire's resistance (and thus indirectly its temperature) the wind speed can be calculated.
Wind speed is important to air navigation. In an aicraft, wind speed relative to the ground can be calculated by using using on-board instruments to measure the wind speed relative to the aircraft, and the plane's speed relative to the ground. Such reports can be used to confirm wind speed forecasts. The development of accurate electronic navigation systems, including inertial navigation and GPS enable this calculation to be done automatically. Modern GPS and inertial systems often include a direct readout of the current wind speed and direction.
Wind speed can also be estimated using radar to measure the doppler shift due to velocity of air. This method is now used very often by meteorologists.
[edit] Speed and velocity
Technically, wind speed is given by
,
where u, v, and w are zonal, meridional, and vertical components of wind velocity. Except in unusual circumstances (e.g. in cumulus updrafts), the vertical component of the velocity is much smaller than the horizontal components.
[edit] Use in aviation
In aviation, wind speed is used to convert between ground speed and true airspeed.
This relationship means that windspeed can be calculated by comparing airspeed (from pitot-based instruments) with ground speed (from a GPS, INS or similar). When the GPS is operating, some aircraft will display the windspeed symbol on their navigation displays.
[edit] Factors affecting wind speed
Wind speed is affected by a number of factors, situations, operating on varying scales (from micro to macro scales). These include the pressure gradient, Rossby waves and jet streams and local weather conditions. There are also links to be found between wind speed and wind direction, notably with the pressure gradient and surfaces that the air is to be found over.
Pressure gradient is a term to describe the difference in air pressure between two points in the atmosphere or on the surface of the Earth. It is vital to wind speed, because the greater the difference in pressure, the faster the wind flows (from the high to low pressure) to balance out the variation. The pressure gradient, when combined with the Coriolis Effect and friction, also influences wind direction
Rossby waves are strong winds in the upper troposphere. These operate on a global scale and move from West to East (hence being known as Westerlies). The Rossby waves are themselves a different wind speed to what we experience in the lower troposphere.
Local weather conditions play a key role in influencing wind speed, as the formation of hurricanes, monsoons or cyclones as freak weather conditions can drastically affect the velocity of the wind.
[edit] Highest speed
The highest surface wind speed ever officially recorded is 372 km/h (231 mph) at the Mount Washington (New Hampshire) Observatory in the US on 12th April 1934, using a heated wire anemometer. The anemometer was later tested by the US National Weather Bureau and confirmed to be accurate.
A higher windspeed recorded at 380 km/h (236 mph) during Typhoon Paka in 1997 in Guam was declared invalid because the instrument was damaged during the storm and could not later be checked for accuracy.
Windspeeds within certain atmospheric phenomena (such as tornadoes) may greatly exceed this value but have never been accurately measured. The figure of 509 km/h (316 mph) during the F5 tornado, Moore in Oklahoma, USA is often quoted as the highest surface wind speed but was measured 30 m (100 feet) above ground.
[edit] See also
