High pressure area

A high pressure area (also called a high or high-pressure) is a region where the atmospheric pressure is greater than surrounding areas. In the northern hemisphere high pressure areas move clockwise, whereas they move counter-clockwise in the southern hemisphere. 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 by adiabatic heating. Thus, high pressure typically brings clear skies. During the day, since no clouds are present to reflect sunlight or reflected inki, 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. As the hot air rises it cools, losing moisture; it is then transported poleward where it descends, creating the high pressure area. 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, because of 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. In the United States the Azores High, also known as the Bermuda High, 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.

source: http://en.wikipedia.org/wiki/High_(atmospheric)