COLD FRONTS ALOFT

miles ahead of the front. These clouds are all found in the warm air. Generally, unless the cold air is unstable and descending currents are weak, there are few clouds in the cold air behind the front. Showers and thunderstorms occur along and just ahead of the front. The ceiling is low only in the vicinity of the front. Visibility is poor during precipitation but improves rapidly after the frontal passage. Upper Air Characteristics Because of the sinking motion of the cold air behind the front and the resultant adiabatic warming, the temperature change across the front is often destroyed or may even be reversed. A sounding taken in the cold air immediately behind the surface front indicates only one inversion and an increase in moisture through the inversion. Farther back of the front, a double inversion structure is evident. The lower inversion is caused by the subsidence effects in the cold air. This is sometimes confusing to the analyst because the subsidence inversion is usually more marked than the frontal inversion and may be mistaken for the frontal inversion. In contrast to the slow-moving cold front, the wind above the fast-moving cold front exhibits only a slight backing with height of about 20 degrees between 950 and 400 mb; the wind direction is inclined toward the front at an average angle of about 45 degrees. The wind components normal and parallel to the front increase with height; the wind component normal to the front exceeds the mean speed of the front at all levels above the lowest layers. On upper air charts, the isotherms are NOT parallel to the front. Instead they are at an angle of about 30 degrees to the front, usually crossing the cold front near its junction with the associated warm front. SECONDARY COLD FRONTS 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. COLD FRONTS ALOFT 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 4-33 shows an example of this type of front and the synoptic structure. Weather from cold 4-35 FRONTAL ZONE AG5f0433 WEST COOL mP WARM mT cP VERY COLD ROCKY MOUNTAINS EAST Figure 4-33.—Cold front aloft.