Pressure Systems (upper level)

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Air pressure is a measure (in millibars) of the force exerted by air on every point of Earth’s surface. The concept of pressure system refers to a region of the atmosphere where air pressure is particularly high or low with reference to those of the surrounding parcels of air.
Weather maps usually show the presence of pressure systems depicting them with a red L, in the case of low pressure systems (almost always related to bad weather conditions), and with a blue H, in the case of high pressure systems (almost invariably related to fine weather conditions). Isobars are the lines that match points of equal pressure; the closer they are, the greater the steepness of the pressure gradient force. Pressure systems are constantly being born and dying, due to thermodynamic interactions within the atmosphere, and with different bodies of water. Some of them tend to persist at a particular location throughout the year, becoming semi-permanent. When low pressure systems adopt an elongated form along an axis, we are before a trough. When high pressure systems form a line or axis, a ridge is born. Low pressure conditions favor the creation of fronts (generated in the boundary zone between air masses of different characteristics), while high pressure conditions favor their dissipation.

Basic concepts

Pressure system: “individual cyclonic-scale feature of atmospheric circulation commonly used to denote either a high or a low, less commonly a ridge or trough”. Cyclone - also named as depression, or [...]

Internal dynamic of highs and lows

Air rotates around highs in an anticyclonic (clockwise in the Northern Hemisphere) direction while spiraling outward from the high pressure center. Such spreading out of the air is known as [...]

Semipermanent Highs and Lows

They are areas of high pressure (anticyclones) and low pressure (cyclones) that tend to persist at a particular latitude belt throughout the year, averaged over long periods of time. Semi-permanent [...]

Creation of fronts

A front is the relatively sudden transition zone between air masses. Fronts are not permanent features; they form and intensify in a process known as frontogenesis. The arrival of a new [...]

Troughs & Ridges

A ridge is an axis or line of high atmospheric pressure, depicted on a weather map as a rise in an isobar; a system of nearly parallel isobars, approximately u-shaped, [...]