A high, or a high pressure area, is a region where the atmospheric pressure is greatest with relation to the surrounding area. 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, which may accompany temperature extremes in both summer and winter, due to the lack of significant cloud cover. This allows for more incoming shortwave solar radiaion and higher temperatures, since no clouds are present to reflect sunlight. At night, the lack of the absorbtive effect of clouds on outgoing longwave radiaion (i.e. heat energy from the surface) is absent, which allows for 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 circulations. Many of the world's deserts are associated with these climatological high pressure systems.
Surface high pressure systems will tend to be larger in area and have weaker surface winds than a given low pressure system, due to the addition of surface friction to the pressure gradient and coriolis effect that drives the circulation.
See also