Wind directionis generally reported by the direction from which thewindoriginates. For example, anorthornortherlywindblows from thenorthto thesouth;[1]the exceptions are onshore winds (blowing onto the shore from the water) and offshore winds (blowing off the shore to the water). Wind direction is usually reported incardinal (or compass) direction,or indegrees.Consequently, a wind blowing from the north has a wind direction referred to as 0° (360°); a wind blowing from the east has a wind direction referred to as 90°, etc.
Weather forecaststypically give the direction of the wind along withits speed,for example a "northerly wind at 15 km/h" is a wind blowingfromthe north at a speed of 15km/h.[1]Ifwind gustsare present, their speed may also be reported.
Measurement techniques
editA variety of instruments can be used to measure wind direction, such as theanemoscope,windsock,andwind vane.[2]All these instruments work by moving to minimize air resistance. The way a weather vane is pointed by prevailing winds indicates the direction from which the wind is blowing.[3]The larger opening of a windsock faces the direction that the wind is blowing from; its tail, with the smaller opening, points in the same direction as the wind is blowing.[4]
Modern instruments used to measure wind speed and direction are calledanemoscopes,anemometersandwind vanes.These types of instruments are used by the wind energy industry, both forwind resource assessmentandturbinecontrol. When a high measurement frequency is needed (such as in research applications), wind can be measured by the propagation speed ofultrasoundsignals or by the effect of ventilation on the resistance of a heated wire.[5]Another type of anemometer usespitot tubesthat take advantage of the pressure differential between an inner tube and an outer tube that is exposed to the wind to determine the dynamic pressure, which is then used to compute the wind speed.[6]
In situations where modern instruments are not available, an index finger can be used to test the direction of wind.[7]This is accomplished by wetting the finger and pointing it upwards. The side of the finger that feels "cool" is (approximately) the direction from which the wind is blowing. The "cool" sensation is caused by an increased rate of evaporation of the moisture on the finger due to the air flow across the finger, and consequently the "finger technique" of measuring wind direction does not work well in either very humid or very hot conditions. The same principle is used to measure thedew pointusing asling psychrometer(a more accurate instrument than the human finger).
Another primitive technique for measuring wind direction is to take a pinch of grass and drop it; the direction that the grass falls is the direction the wind is blowing. This last technique is often used by golfers because it allows them to gauge the strength of the wind.[8]
See also
edit- Air masses
- Apparent wind
- Beaufort scale
- Wind fetch
- Wind power
- Wind rose
- Wind transducer
- Yamartino methodfor calculating the standard deviation of wind direction
References
edit- ^ab"Origin of Wind".National Weather Service.
- ^Myer Kutz (1 December 2015).Handbook of Measurement in Science and Engineering.John Wiley & Sons. pp. 737–.ISBN978-1-118-44697-3.
- ^Frederick K. Lutgens; Edward J. Tarbuck (1989).The Atmosphere: An Introduction to Meteorology.Prentice Hall.ISBN978-0-13-050196-7.
- ^Glossary of Meteorology (2009)."Wind vane".American Meteorological Society. Archived fromthe originalon 2007-10-18.Retrieved2009-03-17.
- ^Glossary of Meteorology (2009)."Anemometer".American Meteorological Society. Archived fromthe originalon 2011-06-06.Retrieved2009-03-17.
- ^Glossary of Meteorology (2009)."Pitot tube".American Meteorological Society. Archived fromthe originalon 2012-05-14.Retrieved2009-03-17.
- ^Raymond A. Serway; John W. Jewett (1 January 2018).Physics for Scientists and Engineers.Cengage Learning. pp. 533–.ISBN978-1-337-67171-2.
- ^Chi Chi Rodriguez (8 May 1975).Everybody's Golf Book.Viking Press.ISBN9780670300365.