More common is an occluded front, where cold and warm fronts overlap, so that the lower-level cold front shuts off (i.e. 'occludes') the upper warm front. This situation is illustrated in Figure 13.9. It is triggered by a poleward kink in the roughly east-west polar front (Figure 12.12), the boundary between a relatively warm northwesterly moving at 7 m/s, say, on the equatorward side, and colder southwesterlies on the poleward side blowing at 13 m/s, for instance (Section 12.3). The two differing air masses ruffle each other, amplifying the kink. The Bergen School explained what happens next by regarding the kink, in effect, as the pivot of scissors, with an advancing cold front as the curved blade to the west, and a slower warm front also swinging clockwise to the east of the kink. The faster cold front catches up on the warm front, closing the scissors, and then overtakes it, as shown in the cross-section AB in Figure 13.9. The occluded front lies at point C. The depth of warm air, i.e. of lightweight air, is shown as greatest above that point, so the surface pressure is lowest there in this cross-section.
An occluded front involves a tongue of relatively warm, moist air lifted on both sides by cold air (Figure 13-9), with heavy rainfalls nearby as the warm, moist air rises. There is eventually a mixing of the warm and cold air masses, which diffuses the temperature difference across the polar front locally and therefore the fronts. Dissipation is complete some four to seven days after the original kink. The regularity of this pattern of events makes
midlatitude weather usefully predictable over a period of a few days (Chapter 15).
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