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A high-pressure area, high or anticyclone is a region where the atmospheric pressure at the surface of the planet is greater than its surrounding environment. Winds within high-pressure areas flow outward from the higher pressure areas near their centers towards the lower pressure areas further from their centers. Gravity adds to the forces causing this general movement, because the higher pressure compresses the column of air near the center of the area into greater density – and so greater weight compared to lower pressure, lower density, and lower weight of the air outside the center. However, because the planet is rotating underneath the atmosphere, and frictional forces arise as the planetary surface drags some atmosphere with it, the air flow from center to periphery is not direct, but is twisted due to the Coriolis effect, or the merely apparent force that arise when the observer is in a rotating frame of reference. Viewed from above this twist in wind direction is in the same direction as the rotation of the planet. The strongest high-pressure areas are associated with cold air masses which push away out of polar regions during the winter when there is less sun to warm neighboring regions. These Highs change character and weaken once they move further over relatively warmer water bodies. Somewhat weaker but more common are high-pressure areas caused by atmospheric subsidence, that is, areas where large masses of cooler drier air descend from an elevation of 8 to 15 km after the lower temperatures have precipitated out the lighter water vapor. (H2O is about half of the molecular weight of the other two main constituents of the atmosphere—Oxygen, O2, and Nitrogen, N2.) Many of the features of Highs may be understood in context of middle- or meso-scale and relatively enduring dynamics of a planet's atmospheric circulation. For example, massive atmospheric subsidences occur as part of the descending branches of Ferrel cells and Hadley cells. Hadley cells help form the subtropical ridge, steer tropical waves and tropical cyclones across the ocean and is strongest during the summer. The subtropical ridge also helps form most of the world's deserts. On English-language weather maps, high-pressure centers are identified by the letter H. Weather maps in other languages may use different letters or symbols. ==Wind circulation in the northern and southern hemispheres== The direction of wind flow around an atmospheric high-pressure area and a low-pressure area, as seen from above, depends on the hemisphere. The scientific terms in English used to describe the weather systems generated by highs and lows were introduced in the mid-1800s, mostly by the British. The scientific theories which explain the general phenomena originated about two centuries earlier. The term Cyclone was coined by Henry Piddington of the British East India Company to describe an especially destructive storm in Mauritius, during February 1845.〔(【引用サイトリンク】url=http://dictionary.reference.com/browse/cyclone )〕 A cyclone forms around a low-pressure area. Anticyclone, the term for the kind of weather around a high-pressure area, was coined in 1877 by Francis Galton〔| "Word Origin & History" ()|accessed 2013-01-24〕 to indicate an area whose winds revolved in the opposite direction of a cyclone. In British English, the opposite direction of clockwise is referred to as anticlockwise, making the label ''anticyclones'' a logical extension. A simple rule of thumb is that for high-pressure areas, where generally air flows from the center outward, the coriolis force given by the earth's rotation to the air circulation is in the opposite direction of earth's apparent rotation if viewed from above the hemisphere's pole. So, both the earth and winds around a low-pressure area rotate counter-clockwise in the northern hemisphere, and clockwise in the southern. The opposite to these two cases occurs in the case of a high. These results derive from the Coriolis effect; that article explains in detail the physics, and provides an animation of a model to aid understanding. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「High-pressure area」の詳細全文を読む スポンサード リンク
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