Sometimes there is a tendency for a trough of low pressure to form to the rear of a cold front, and a secondary cold front may develop in this trough. Secondary cold fronts usually occur during outbreaks of very cold air behind the initial outbreak. Secondary cold fronts may follow in intervals of several hundred miles to the rear of the rapidly moving front. When a secondary cold front forms, the primary front usually tends to dissipate and the secondary front then becomes the primary front. Secondary fronts usually do not occur during the summer months because there is rarely enough temperature discontinuity.

There are two types of upper cold fronts. One is the upper cold front associated with the warm occlusion that is discussed later in this unit. The other occurs frequently in the areas just east of mountains in winter. This cold front aloft is associated with mP air crossing the mountains behind a cold front or behind a cold trough aloft and a very cold layer of continental polar air lying next to the ground over the area east of the mountains. The area east of the Rocky Mountains is one such area in the United States. When warm maritime tropical air has moved northward from the Gulf of Mexico and has been forced aloft by the cold cP air, and cool mP air flows over the mountains, it forces its way under the warm mT air aloft. The resulting front then flows across the upper surface of the colder cP air just as if it were the surface of the ground. All frontal activity in this case takes place above the top of the cP layer. Figure 4-3-3 shows an example of this type of front and the synoptic structure.

Weather from cold fronts aloft can produce extensive cloud decks and blizzard conditions for several hundred miles over the Midwestern plains.

Learning Objective: Describe instability and squall lines and their relationship to cold fronts.

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