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Secondary Cold Fronts

In the last stages of an extratropical cyclone, there is a tendency for a trough to form to the rear of the primary frontal trough. A secondary front may occur in this trough. Secondary cold fronts occur within cold air masses and may follow in intervals of several hundred miles to the rear of rapidly moving cold fronts. Usually, as a secondary cold front develops, the primary front tends to dissipate. The secondary front then becomes the primary front. Secondary fronts are far more common during the winter when temperature discontinuity is greatest.

Warm Fronts

Active warm fronts are generally located in pressure troughs on the surface charts. The troughs are not as pronounced as those ob-served with cold fronts; therefore, other meteorological elements are used in determining their location.

1. Pressure tendencies. Pressure usually falls for an appreciable length of time before the fron-tal passage. Normally, it is steady after passage. The tendencies in advance of the front are therefore \ (steady or unsteady fall). A warm frontal passage is usually indicated by a \ tendency.

2. Wind. The wind in advance of a warm front in the Northern Hemisphere is usually from the southeast, shifting to southwest after passage. The wind speed normally increases as the front approaches. The wind shift accompanying a warm front is seldom as abrupt as with a cold front.

3. Cloud forms. Warm fronts are nearly always well defined by typical stratified clouds. They are generally cirrus, cirrostratus, altostratus, nimbostratus, and stratus, with the cirrus appear-ing as much as 1,000 miles (or more) before the actual surface passage. The cloud types that form after passage of the warm front are typical of the warm air mass.

4. Precipitation. The precipitation area of warm fronts extends about 300 miles in advance of the surface front. Precipitation occurs mainly in the form of continuous or intermittent rain, snow, or drizzle. However, when the warm air is connectively unstable, showers and thunderstorms may occur in addition to the steady precipitation.

5. Temperature and dew-point changes. Abrupt temperature changes, like those charac-teristic of cold fronts, do not accompany the warm frontal passage. Instead, the temperature change is gradual. It starts increasing slowly with the approach of the front and increases slightly more rapidly with the passage. The dew point is normally observed to rise as the front approaches, and a further increase follows the frontal passage when the air in the warm sector is of maritime origin.

6. Visibility and ceiling. The visibility and ceil-ings are normally good until the precipitation begins. Then they decrease rapidly. Dense fog fre-quently occurs in advance of a warm front. An improvement is experienced after passage. The parameters for locating a warm front are not as distinct as other fronts; therefore, the warm front is very difficult to locate at times.

Occluded Fronts

Because the occlusion is a combination of a cold front and a warm front, the resulting weather is a combination of conditions that exist with both fronts. Ahead of a cold-type occlusion, as the warm air is lifted, all the clouds associated with a warm front are found producing typical prefron-tal precipitation extensively for a distance of 250 to 300 miles. Typical cold frontal weather is found throughout the narrow belt in the vicinity of the surface front. However, the thunderstorms are less intense than those of a typical cold front, since the source of warm air has been cut off from the surface and the energy received comes only from the warm air trapped aloft. Instability showers often follow the cold front when the cold air is unstable. The most violent weather occurs on the upper front for a distance of 50 to 100 miles north of the northern tip of the warm sector. After the occlusion has passed, the weather usually clears rapidly.

The weather associated with the warm-type occlusion is very similar to that of the cold-type occlusion. With the warm-type occlusion, the high-level thunderstorms associated with the upper cold front develop quite some distance ahead of the surface front (up to 200 miles), and the weather band, in general, is wider (up to 400 miles). The air behind the cold front, flowing up the warm frontal surface, causes cumuliform-type clouds to form. In this area, precipitation and severe icing may be found. The most violent weather occurs on the upper front, 50 to 100 miles north of the northern tip of the warm sector.

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