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MIGRATORY SYSTEMS

General circulation, based on an average of wind conditions, is a more or less quasi-stationary circulation. Likewise, much of the secondary cir-culation depends on more or less static conditions that, in turn, depend on permanent and semiper-manent high- and low-pressure areas. Changes in the circulation patterns discussed so far have been largely seasonal. However, secondary circulation also includes wind systems that migrate con-stantly, producing rapidly changing weather conditions throughout all seasons, especially in the middle latitudes. The migratory circulation systems are as- sociated with air masses, fronts, cyclones, and at-anticyclones. These are covered in detail in the next unit. 

Anticyclones

An anticyclone (high) is an area of relatively high pressure with a clockwise flow (wind circula-tion) in the Northern Hemisphere and counter-clockwise flow in the Southern Hemisphere. The windflow in an anticyclone is slightly across the isobars and away from the center of the an-ticyclone. (See fig. 3-2-2.) Anticyclones are com-monly called highs or high-pressure areas. 

The formation of an anticyclone or the inten-sification of an existing one is called AN-TICYCLOGENESIS. Anticyclogenesis refers to the development of anticyclonic circulation as well as the intensification of an existing anticyclonic flow. When a high-pressure center is increasing in pressure, the high is BUILDING or INTEN-SIFYING. Although a high can build (or inten-sify) without an increase in anticyclonic flow, it is rare. Normally, building and anticyclogenesis occur simultaneously.

The weakening of anticyclonic circulation is ANTICYCLOLYSIS. When the pressure of a high is decreasing, we say the high is weaken-ing. Anticyclolysis and weakening can occur separately, but usually occur together. The vertical extent of pressure greatly depends on the air temperature. Since density increases with a decrease in temperature, pressure decreases more rapidly vertically in colder air than in warmer air.

In a cold anticyclone (such as the Siberian high), the vertical extent is shallow; while in a warm anticyclone (such as the subtropical high), the vertical extent reaches high into the upper atmosphere due to the slow decrease in temperature with elevation.

Cyclones 

A cyclone (low) is a circular or nearly circular area of low pressure with a counterclockwise flow. The flow is slightly across the isobars toward the center in the Northern Hemisphere and clockwise in the Southern Hemisphere. (See fig. 3-2-3.) It is commonly called a low or a depression. This use of the word cyclone should be distinguished from the colloquial use of the word as applied to the tornado or tropical cyclone (hurricane). The formation of a new cyclone or the inten-sification of the cyclonic flow in an existing one is called CYCLOGENESIS. When the pressure in the low is falling, we say the low is deepening. Cyclogenesis and deepening can also occur separately, but usually occur at the same time. The decrease or eventual dissipation of a cyclonic flow is called CYCLOLYSIS. When the pressure in a low is rising, we say the low is fill-ing. Cyclolysis and filling usually occur simultaneously.

Cyclones in middle and high latitudes are re-ferred to as extratropical cyclones. The term cyclone refers to hurricanes and typhoons.

Figure 3-2-2.—Anticyclone.

Figure 3-2-3.—Cyclone.

Learning Objective: Determine how the vertical structure of secondary circulations is affected by the mean temperature and the vertical spacing of isobars.

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