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High pressure area
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A high-pressure area (also called a high or a high-pressure) 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 due to the higher density air near their center and friction with land. Due to the coriolis force, winds flow clockwise around high-pressure systems in the northern hemisphere and counter-clockwise (BE: anticlockwise) in the southern hemisphere.
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Encyclopedia
A high-pressure area (also called a high or a high-pressure) 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 due to the higher density air near their center and friction with land. Due to the coriolis force, winds flow clockwise around high-pressure systems in the northern hemisphere and counter-clockwise (BE: anticlockwise) in the southern hemisphere. Regions of high-pressure are alternatively referred to as anticyclones. High-pressure areas are generally associated with cooler, drier air as well as clearing skies due to their formation within areas of atmospheric subsidence, or areas of large scale air descent. The strongest high-pressure areas are associated with arctic air masses during the winter, which modify and weaken once they move over relatively warmer water bodies. The area of high pressure associated with the descending branch of the Hadley cell, known as 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 the world's deserts. Arctic high-pressure systems weaken with height, while subtropical ridges strengthen with height. On English weather maps, high-pressure centers are associated with the letter H. On Spanish weather maps, high-pressure centers are associated with the letter A, for alta.
Formation
High-pressure systems form due to downward motion through the troposphere, the atmospheric layer where weather occurs. Preferred areas within a synoptic flow pattern in higher levels of the troposphere are beneath the western side of troughs. On weather maps, these areas show converging winds (isotachs), also known as confluence, or converging height lines near or above the level of non-divergence, which is near the 500 hPa pressure surface about midway up through the troposphere. High-pressure systems are alternatively referred to as anticyclones. On weather maps, high-pressure centers are associated with the letter H in English, or A in Spanish, because alta is the Spanish word for high, within the isobar with the highest pressure value. On constant pressure upper level charts, it is located within the highest height line contour.
Typical conditions
Highs are frequently associated with light winds at the surface and subsidence through the lower portion of the troposphere. Subsidence will generally dry out an air mass by adiabatic, or compressional, heating. Thus, high pressure typically brings clear skies. During the day, since no clouds are present to reflect sunlight, there is more incoming shortwave solar radiation and temperatures rise. At night, the absence of clouds means that outgoing longwave radiation (i.e. heat energy from the surface) is not absorbed, giving cooler diurnal low temperatures in all seasons. When surface winds become light, the subsidence produced directly under a high-pressure system can lead to a build up of particulates in urban areas under the ridge, leading to widespread haze. If the low level relative humidity rises towards 100 percent overnight, fog can form.
Strong, vertically shallow high-pressure systems moving from higher latitudes to lower latitudes in the northern hemisphere are associated with continental arctic air masses. Once arctic air moves over an unfrozen ocean, the air mass modifies greatly over the warmer water and takes on the character of a maritime air mass, which reduces the strength of the high-pressure system. When extremely cold air moves over relatively warm oceans, polar lows can develop. However, warm and moist (or maritime tropical) air masses which move poleward from tropical sources are slower to modify than arctic air masses.
In climatology
Climatologically, high pressure forms at the horse latitudes, or torrid zone, between the latitudes of 20 and 40 degrees from the equator, as a result of air which has been uplifted at the equator. As the hot air rises it cools, losing moisture; it is then transported poleward where it descends, creating the high-pressure area. This is part of the Hadley cell circulation and is known as the subtropical ridge or subtropical high, and is strongest in the summer. The subtropical ridge is a warm core high-pressure system, meaning it strengthens with height. Many of the world's deserts are caused by these climatological high-pressure systems.
Some climatological high-pressure areas acquire regionally based names. The land-based Siberian High often remains quasi-stationary for more than a month during the most frigid time of the year, making it unique in that regard. It is also a bit larger and more persistent than its counterpart in North America. Surface winds accelerating down valleys down the western Pacific ocean coastline, causing the winter monsoon. Arctic high-pressure systems such as the Siberian High are cold core, meaning that they weaken with height. The influence of the Azores High, also known as the Bermuda High, brings fair weather over much of the North Atlantic Ocean and mid to late summer heat waves in western Europe. Along its southerly periphery, the clockwise circulation often impels easterly waves, and tropical cyclones which develop from them, across the ocean towards landmasses in the western portion of ocean basins during the hurricane season.
Connection to wind
Wind flows from areas of high pressure to areas of low pressure. This is due to density differences between the two air masses. Since stronger high-pressure systems contain cooler or drier air, the air mass is more dense and flows towards areas that are warm or moist, which are in the vicinity of low pressure areas in advance of their associated cold fronts. The stronger the pressure difference, or pressure gradient, between a high-pressure system and a low pressure system, the stronger the wind. The coriolis force caused by the Earth's rotation is what gives winds within high-pressure systems their clockwise circulation in the northern hemisphere (as the wind moves outward and is deflected right from the center of high pressure) and counterclockwise circulation in the southern hemisphere (as the wind moves outward and is deflected left from the center of high pressure). Friction with land slows down the wind flowing out of high-pressure systems and causes wind to flow more outward, or flowing more ageostrophically, from their centers.
See also
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