<P> On weather maps, these areas show converging winds (isotachs), also known as convergence, near or above the level of non-divergence, which is near the 500 hPa pressure surface about midway up through the troposphere, and about half the atmospheric pressure at the surface . </P> <P> High - pressure systems are alternatively referred to as anticyclones . On English - language weather maps, high - pressure centers are identified by the letter H in English, within the isobar with the highest pressure value . On constant pressure upper level charts, it is located within the highest height line contour . </P> <P> Highs are frequently associated with light winds at the surface and subsidence through the lower portion of the troposphere . In general, subsidence will dry out an air mass by adiabatic, or compressional, heating . Thus, high pressure typically brings clear skies . During the day, since no clouds are present to reflect sunlight, there is more incoming shortwave solar radiation and temperatures rise . 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 . When surface winds become light, the subsidence produced directly under a high - pressure system can lead to a buildup of particulates in urban areas under the ridge, leading to widespread haze . If the low level relative humidity rises towards 100 percent overnight, fog can form . </P> <P> Strong, vertically shallow high - pressure systems moving from higher latitudes to lower latitudes in the northern hemisphere are associated with continental arctic air masses . Once arctic air moves over an unfrozen ocean, the air mass modifies greatly over the warmer water and takes on the character of a maritime air mass, which reduces the strength of the high - pressure system . When extremely cold air moves over relatively warm oceans, polar lows can develop . However, warm and moist (or maritime tropical) air masses that move poleward from tropical sources are slower to modify than arctic air masses . </P>

What conditions are typical of high pressure cells in the northern hemisphere