A pressure system is a region of the Earth's atmosphere where air pressure is unusually high or low. High and low pressures develop (and dissipate) constantly due to thermodynamic interactions of temperature differentials in the atmosphere and water of oceans and lakes.

Pressure systems on weather maps

On weather maps, which often display weather fronts and pressure systems graphically, low-pressure systems are depicted with a capital L, unless they are tropical storms or tropical cyclones (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 isobars, or lines on the map at which pressure is equal.

Low pressure system

Cyclonic properties: in the northern hemisphere, winds around the system move counterclockwise, and in the southern hemisphere they move clockwise. Low pressure systems, additionally, often become junctures of fronts.

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.

Low pressure systems are generally very warm due to the fact the warm air rises making the system it is on having low pressure. This is generally the reason for storms and Tropical cyclones.

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 occur often, but not always, when there is an uneven heating of the ground, causing the hot air to rise and spread out. This rising air creates an air 'void', or a low-pressure system in the area above the ground. However, when the air cools and sinks back down to the ground,a high-pressure system is created. 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 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.