High pressure area
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A high, high-pressure cell, or high pressure area is a region where the atmospheric pressure is greater than the surrounding area. In some countries, these regions may be referred to as anticyclones. Highs are frequently associated with light winds and subsidence. Subsidence will generally evaporate most cloud droplets after less than 500 meters, due to adiabatic heating. Thus, high pressure typically brings clear skies. During the day, since no clouds are present to reflect sunlight or reflected infrared, there is more incoming shortwave solar radiation and temperatures are higher in the summer and lower in the winter. 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.
Climatologically, high pressure forms at the Horse Latitudes as a result of air which has been uplifted at the equator, transported poleward, and cooled. This is also known as Hadley Cell circulation. Many of the world's deserts are associated with these climatological high pressure systems.
Surface high pressure systems tend to be larger in area and have weaker surface winds than low pressure systems, due to the addition of surface friction to the pressure gradient and coriolis effect that drives the circulation.
Some high pressure areas persist long enough that they acquire 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. Citizens of the United States are aware of the Azores High, also known as the Bermuda High, which brings fair weather over much of the North Atlantic Ocean. Along its southerly periphery, the clockwise circulation often impels easterly waves across the ocean towards North America during the hurricane season.
