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9.22 Winds - General


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Pressure and Wind

Wind is the quasi-horizontal movement of air (as opposed to an air current) caused by uneven heating of the Earth's surface. It occurs at all scales, from local breezes generated by heating of land surfaces and lasting tens of minutes to global winds resulting from solar heating of the Earth. The two major influences on the atmospheric circulation are the differential heating between the equator and the poles, and the rotation of the planet (Coriolis Effect).

Given a difference in barometric pressure between two air masses, a wind will arise between the two which tends to flow from an area of high pressure to an area of low pressure until the two air masses are at the same pressure, although these flows will be modified by the Coriolis Effect in the extratropics.

Winds can be classified either by their scale, the kinds of forces which cause them (according to the atmospheric equations of motion), or the geographic regions in which they exist. There are global winds, such as the wind belts which exist between the atmospheric circulation cells. There are upper-level winds, such as the jet streams. There are synoptic-scale winds that result from pressure differences in surface air masses in the middle latitudes, and there are winds that come about as a consequence of geographic features such as the sea breeze. Mesoscale winds are those which act on a local scale, such as gust fronts. At the smallest scale are the winds which blow on a scale of only tens to hundreds of metres and are essentially unpredictable, such as microburst and dust devils.

Horizontally, on the Earth's surface wind always blows from areas of high pressure to areas of low pressure, usually at speeds determined by the rate of air pressure change between pressure centres. This situation is comparable to someone skiing down a hill. The skier will of course move from the top of the hill to the bottom of the hill, with the speed of their descent controlled by the gradient or steepness of the slope. Likewise, wind speed is a function of the steepness or gradient of atmospheric air pressure found between high and low pressure systems. When expressed scientifically, pressure change over a unit distance is called pressure gradient force and the greater this force the faster the winds will blow.

On weather maps, pressure is indicated by drawing equal lines of pressure, called isobars, at regular 4 hectopascal intervals (e.g., 996 hPa, 1000 hPa, 1004 hPa, 1008 hPa). If the isobars are closely spaced, we can expect the pressure gradient force to be great, and wind speed to be high. In areas where the isobars are spaced widely apart, the pressure gradient is low and light winds normally exist. High speed winds develop in areas where isobars are closer.

Buys Ballot: was a Dutch chemist and meteorologist after whom Buys-Ballot's law and the Buys Ballot table are named. Buys Ballot is best known for his accomplishments in the field of meteorology, specifically the explanation of the direction of air flow in large weather systems.

Buys-Ballot's law states that if a person in the Southern Hemisphere stands with their back to the wind, the low atmospheric pressure is found to their right. His main research effort in meteorology went into examining long time series for regularities.