Wind speed

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	calm (0–2 kn)
	3–7 kn
	8–12 kn
	13–17 kn
	18–22 kn
	23–27 kn
	28–32 kn
	33–37 kn
	38–42 kn
	43–47 kn
	48–52 kn
	53–57 kn
	58–62 kn
	63–67 kn
	98–102 kn
	102–107 kn

Wind speed is the speed of movement of air relative to a fixed point on the earth. Wind is a term applied when talking about the movement of air from one place to the next.

Wind speed usually means the movement of air in an outside environment, but the speed of movement of air 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 for a discussion of this.

1 Measurement
2 Speed and velocity
3 Use in aviation
4 Factors affecting wind speed

[edit] Measurement

Wind speed may be measured by a variety of tools. The anemometer, consisting of a rotating vane, has been the most common method of measuring speeds close to the ground for hundreds of years. They can fix his position relative to the ground and knowing from instruments his movement through the air can estimate the wind speed and direction over the time he has been flying. 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 windspeed and direction.

[edit] Speed and velocity

Technically, wind speed is given by

$|\mathbf{v}|=\sqrt{u^2+v^2+w^2}$ ,

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, 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.