Certain regions of the world exhibit a high frequency of frontogenesis. These regions are coincident with the greatest temperature contrasts. Two of the most important frontal zones are those over the north Pacific and north Atlantic Oceans. In winter, the arctic front, a boundary between polar and arctic air, forms in high latitudes over northwest North America, the north Pacific, and near the Arctic Circle north of Europe (fig. 4-2-6). In summer, the arctic front mainly disappears, ex-cept north of Europe. (See fig. 4-2-7.)

The polar front, on the other hand, is present the year round, although it is not as intense in the summer as in the winter because of a lessening temperature contrast between the opposing air masses. The polar front forms wherever the wind flow and temperature contrast is favorable. Usually this is the boundary between tropical and polar air, but it may form between maritime polar and continental polar air. It also may exist between modified polar air and a fresh outbreak of polar air. The polar front is common over North America in the continental regions in winter in the vicinity of 50N latitude.

The polar front in winter is found most frequently off the eastern coasts of continents in areas of 30 to 60 latitude. It is also found over land; but since the temperature contrasts are greater between the continent and the oceans, especially in winter, the coastal areas are more favorable for formation and intensification of the polar front. 

The intertropical convergence zone (ITCZ), though not truly a front but a field of convergence between the opposing trades, forms a third semipermanent frontal type. This region shows a seasonal variation just as do the trade winds.

Learning Objective: Describe the frontal elements and general characteristics of fronts.

Figure 4-2-6.Chart showing world air masses, fronts and centers of major pressure systems in January.

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