Information about Wind Speed

	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 a term applied when talking about the movement of air from one place to the next.

Please note, "Wind speed" should not be confused with the Speed of Wind. Analogous to the Speed of Sound or the Speed of Light, the Speed of Wind, at 3.1 meters/second, is a constant used throughout the scientific community in Wind speed calculations. The most popular theories suggest the Speed of Wind is a direct result of the Earth's rotation combined with additional atmospheric and gravitational factors. It should also be noted that the Speed of Wind remains a constant 3.1 m/s at any point on the Earth's surface.

Wind speeds usually mean 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.

Measurement

The most elementary method of measuring wind speed is to estimate the speed from observed phenomena, this is the basis of the Beaufort scale, for example, zero on the Beaufort scale can be recognised by vertically rising smoke. Obviously this method is not the most accurate available so each value on the scale represents a range of values, three on the scale for example covers wind speeds between seven and ten knots.

The anemometer was the most accurate method of measuring wind speeds close to the ground for hundreds of years. 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 and a pilot can fix their position relative to the ground by using on board instruments, then use this information to estimate the wind speed and direction over time. 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.

Using radar to measure the doppler shift due to velocity of air can give an estimate of speed of wind and this is now a method used very frequently by meteorologists.

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.

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.

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.

Highest Speed

The highest surface wind speed ever officially recorded is 372 km/h (231 mph) during a gust 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 windspeed but was measured by Doppler radar (which is only able to provide an estimate) and was measured 30 m (100 feet) above ground.