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, anti-clockwise 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.