In meteorology, wind speed, or wind flow speed, is a fundamental atmosphere quantity caused by air moving from high to low pressure, usually due to changes in temperature. Wind speed is now commonly measured with an anemometer.
Wind speed affects weather forecasting, aviation and maritime operations, construction projects, growth and metabolism rates of many plant species, and has countless other implications. Wind direction is usually almost parallel to isobars (and not perpendicular, as one might expect), due to Earth's rotation.
Units
For historical reasons, other units such as miles per hour (mph), knots (kn), Measuring Wind Speed in Knots "The reason why sea winds are measured in knots at all has to do with maritime tradition" and feet per second (ft/s) are also sometimes used to measure wind speeds. Historically, wind speeds have also been classified using the Beaufort scale, which is based on visual observations of specifically defined wind effects at sea or on land.
Factors affecting wind speed
The Pressure gradient describes 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 from that experienced in the lower troposphere.
Local weather conditions play a key role in influencing wind speed, as the formation of hurricanes, , and cyclones as freak weather conditions can drastically affect the flow velocity of the wind.
Highest speed
Non-tornadic
Currently, the second-highest surface wind speed ever officially recorded is at the Mount Washington (New Hampshire) Observatory above sea level in the US on 12 April 1934, using a hot-wire anemometer. The anemometer, specifically designed for use on Mount Washington, was later tested by the US National Weather Bureau and confirmed to be accurate.
Tornadic
On other planets
Measurement
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Acoustic resonance wind sensors are a variant of the ultrasonic sensor. Instead of using time of flight measurement, acoustic resonance sensors use resonating acoustic waves within a small purpose-built cavity. Built into the cavity is an array of ultrasonic transducers, which are used to create the separate standing-wave patterns at ultrasonic frequencies. As wind passes through the cavity, a change in the wave's property occurs (phase shift). By measuring the amount of phase shift in the received signals by each transducer, and then by mathematically processing the data, the sensor is able to provide an accurate horizontal measurement of wind speed and direction.Kapartis, Savvas (1999) "Anemometer employing standing wave normal to fluid flow and travelling wave normal to standing wave"
Another tool used to measure wind velocity includes a GPS combined with pitot tube. A fluid flow velocity tool, the Pitot tube is primarily used to determine the air velocity of an aircraft.
Design of structures
In the United States, the wind speed used in design is often referred to as a "3-second gust", which is the highest sustained gust over a 3-second period having a probability of being exceeded per year of 1 in 50 (ASCE 7-05, updated to ASCE 7-16). This design wind speed is accepted by most building codes in the United States and often governs the lateral design of buildings and structures.
In Canada, reference wind pressures are used in design and are based on the "mean hourly" wind speed having a probability of being exceeded per year of 1 in 50. The reference dynamic pressure is calculated using the equation , where is the air density and is the wind speed.NBC 2005 Structural Commentaries – Part 4 of Div. B, Comm. I
Historically, wind speeds have been reported with a variety of averaging times (such as fastest mile, 3-second gust, 1-minute, and mean hourly) which designers may have to take into account. To convert wind speeds from one averaging time to another, the Durst Curve was developed, which defines the relation between probable maximum wind speed averaged over some number of seconds to the mean wind speed over one hour.ASCE 7-05 commentary Figure C6-4, ASCE 7-10 C26.5-1
See also
- American Society of Civil Engineers (promulgator of ASCE 7-05, current version is ASCE 7-16)
- Beaufort scale
- Fujita scale and Enhanced Fujita Scale
- International Building Code (promulgator of NBC 2005)
- ICAO recommendations – International System of Units
- Knot (unit)
- Prevailing wind
- Saffir–Simpson Hurricane Scale
- TORRO scale
- Wind direction
External links
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