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Pressure systems
A pressure system is a region of the Earth's atmosphere where air pressure is unusually
high or low. High and low pressures form (and die) constantly due to thermodynamic
interactions of the atmosphere and water in oceans, lakes, and other bodies of water.
Low pressure system
The components of storms are attracted to regions of low pressure. For this reason, heavy
precipitation and overcast conditions are often associated with low-pressure systems. Due
to the Coriolis Effect, low-pressure systems often develop cyclonic properties: in the
northern hemisphere, winds around the system move counter clockwise, and in the
southern hemisphere they move clockwise. Low pressure systems, additionally, often
become junctures of fronts. You may have seen a weather map with a red L on it. This
red L means there is a low pressure system over that area of the map.
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. Tornadoes invariantly
have very strong local low-pressure systems at their vortices.
Air moves into a Low pressure system. It pushes any air that was there upwards.
High-pressure system
High pressure systems are associated with clear, cool weather. Around high-pressure
systems, winds flow clockwise in the northern hemisphere, counter clockwise in the
southern hemisphere. You may have seen a weather map with a blue H on it. This blue H
means there is a high pressure system over that area of the map.
In the northern winter, high-pressure systems (called Canadian highs or Arctic air
masses) often migrate to mid-latitude regions such as the North American upper
Midwest, New England, and northern Europe. These create cold snaps 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 (45°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 snowstorms.
High pressure usually means good weather.
The air moving away from the High pressure system leaves a "hole" to be filled, so
air from above sinks into that "hole".