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PRESSURE SYSTEMS ON WEATHER MAPS On Weather Maps , which often display Weather Front s and pressure systems graphically, low-pressure systems are depicted with a capital L, unless they are Tropical Storm s or Tropical Cyclone s (in which case the symbols for those weather systems are used). High-pressure systems are depicted with an '''H'''. The steepness of the pressure Gradient can be observed according to the density of Isobar s, or lines on the map at which pressure is equal. LOW PRESSURE SYSTEM The components of , winds around the system move counterclockwise, and in the Southern Hemisphere they move clockwise. Low pressure systems, additionally, often become junctures of fronts. Most of history's most powerful storms, such as the 1993 North American Storm Complex and all tropical cyclones, have been low-pressure cyclonic systems. Tornado es invariantly have very strong local low-pressure systems at their Vortices . The lowest recorded non-tornadic barometric pressure was 869.96 millibars (25.69 inches of mercury), occurred in the Western Pacific during Typhoon ''Tip'' on 12 October , 1979 . HIGH-PRESSURE SYSTEM High pressure systems are associated with clear, cool weather. Around high-pressure systems, winds flow clockwise in the northern hemisphere, counterclockwise in the southern hemisphere. In the northern Winter , high-pressure systems (called ''Canadian highs'' or ''Arctic air masses'') often migrate to midlatitude regions such as the North America n Upper Midwest , New England , and northern Europe . These create Cold Snap s where unseasonably cold and sunny weather are observed. Cold snaps often follow winter warm spells, where temperatures may be as high as 10°C to 20°C (50-68 °F), and often happen suddenly. The most dramatic Arctic cold snaps, observed in the central regions of North America involve temperature drops of 25°C (77°F) or more in a few hours. Arctic highs, alone, rarely trigger precipitation because of the cloudless weather they produce. However, in combination with other weather-making systems, the cold air they bring can produce massive Snowstorm s. High pressure usually means good weather. |
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