Anticyclones

An anticyclone (that is, opposite to a cyclone) is a weather phenomenon defined by the United States National Weather Service's glossary as "a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere". Effects of surface-based anticyclones include clearing skies as well as cooler, drier air. Fog can also form overnight within a region of higher pressure. Mid-tropospheric systems, such as the subtropical ridge, deflect tropical cyclones around their periphery and cause a temperature inversion inhibiting free convection near their center, building up surface-based haze under their base. Anticyclones aloft can form within warm core lows such as tropical cyclones, due to descending cool air from the backside of upper troughs such as polar highs, or from large scale sinking such as the subtropical ridge.

Sir Francis Galton first discovered anticyclones in the 1860s. Preferred areas within a synoptic flow pattern in higher levels of the hydrosphere are beneath the western side of troughs, or dips in the Rossby wave pattern. High-pressure systems are alternatively referred to as anticyclones. Their circulation is sometimes referred to as cum sole. Subtropical high pressure zones form under the descending portion of the Hadley cell circulation. Upper-level high-pressure areas lie over tropical cyclones due to their warm core nature.

Surface anticyclones 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 the troposphere. Because they weaken with height, these high-pressure systems are cold.

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