Wind speed

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, or wind velocity (when directionality is considered), is a fundamental abiotic factor that affects the growth and metabolism of many plant species.[1]

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.

Wind speed is measured with an anemometer.

Contents


Factors affecting wind speed

Wind speed is affected by a number of factors and 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 over which the air is found.

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 from what we experience in the lower troposphere.

Local weather conditions play a key role in influencing wind speed, as the formation of hurricanes, monsoons and cyclones as freak weather conditions can drastically affect the velocity of the wind.

Highest speed

During the passage of Tropical Cyclone Olivia on 10 April 1996, an automatic weather station on Barrow Island, Australia, registered a maximum wind gust of 408 km/h (220 kn; 253 mph).[2] The wind gust was evaluated by the WMO Evaluation Panel who found that the anemometer was mechanically sound and the gust was within statistical probability and ratified the measurement in 2010, however, the wind speed was measured inside of the cyclone and not at ground level.[3] During the cyclone several extreme gusts of greater than 300 km/h (160 kt) were recorded, with a maximum 5 minute mean speed of 176 km/h (95 kt), the extreme gust factor was in the order of 2.27-2.75 times the mean wind speed. The pattern and scales of the gusts suggests that a mesovortex was embedded in the already strong eyewall of the cyclone.[3]

The second highest surface wind speed ever officially recorded is 372 km/h (231 mph) at the Mount Washington (New Hampshire) Observatory in the US on 12 April 1934, using a heated anemometer. The anemometer, specifically designed for use on Mount Washington, was later tested by the US National Weather Bureau and confirmed to be accurate.[4] The highest surface wind speed ever officially recorded in Asia was recorded in Afghanistan on 14 August 2008: 328 km/h (204 mph) in Ab-Paran, Ghowr.

Windspeeds within certain atmospheric phenomena (such as tornadoes) may greatly exceed these values but have never been accurately measured. The figure of 509 km/h (316 mph) during the F5 tornado in Moore, Oklahoma is often quoted as the highest surface wind speed but was measured 30 m (90 feet) above ground.

In 1991, a chase team from the University of Oklahoma chased a tornado in Red Rock, Oklahoma and used a portable Doppler weather radar to measure a wind speed of 460 km/h (286 mph).

According to Alan F. Arbogast ("Discovering Physical Geopgraphy") wind direction and speed are affected by three (3) main factors:

1. Pressure Gradient - the difference in barometric pressure between adjacent zones of high and low pressure.

2. Frictional Forces - features on the Earth's surface which oppose the wind; e.g.: mountains, trees, buildings, etc.

3. Coriolis Effect - the Earth's rotation causes winds to be deflected to the right in the Northern Hemisphere, and in the Southern Hemisphere to the left.

All three of these combined result in the sprial motion of air in both high and low pressure systems. (For more information, see chapter six (6), pp. 126–132)

Design of structures

Wind speed is a common factor in the design of structures and buildings around the world. The wind speed is often the governing factor in the "lateral" design of a structure and is used by professional engineers and designers. 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-latest edition). Windspeedbyzip [5] maps out the design wind speed as suggested by ASCE 7-05 for the United States. This design wind speed is accepted by most building codes in the United States and often times 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 wind pressure (q) is calculated in Pascals using the following equation (ref: NBC 2005 Structural Commentaries - Part 4 of Div. B, Comm. I): q=(1/2)pV**2 where p is the air density in kg/m**3 and V is wind speed in m/s. Historically, wind speeds have been reported with a variety of averaging times (fastest mile, 3-second gust, 1-minute and mean hourly for example) which designers may have to take into account. To convert wind speeds from one averaging time to another, the Durst Curve (Ref: ASCE 7-05 commentary Figure C6-2) was developed which defines the relation between probable maximum wind speed averaged over t seconds, V(t), and mean wind speed over one hour V(3600).

See also

References