OVERVIEW Describe the formation, structure, and charac- teristics of cold and warm occluded fronts.

OUTLINE

An occluded front is a composite of two fronts. They form when a cold front overtakes a warm front and one of these two fronts is lifted aloft. As a result, the warm air between the cold and warm front is shut off. An occluded front is often referred to simply as an occlusion. Occlusions may be either of the cold type or warm type. The type of occlusion is determined by the temperature difference between the cold air in advance of the warm front and the cold air behind the cold front.

A cold occlusion forms when the cold air in advance of a warm front is WARMER than the cold air to the rear of the cold front. The over-taking cold air undercuts the cool air in advance of the warm front. This results in a section of the warm front being forced aloft.

A warm occlusion forms when the air in ad-vance of the warm front is COLDER than the air to the rear of the cold front. When the COLD air of the cold front overtakes the warm front, it moves up over this COLDER air in the form of an upper cold front. 

The primary difference between a warm and a cold type of occlusion is the location of the associated upper front in relation to the surface front (fig. 4-5-1). In a warm type of occlusion, the upper cold front precedes the

Figure 4-5-1.Sketch of occlusions (in the horizontal) and associated upper fronts.

surface-occluded front by as much as 200 miles. In the cold type of occlusion the upper warm front follows the surface-occluded front by 20 to 50 miles. 

Since the occluded front is a combination of a cold front and a warm front, the resulting weather is that of the cold fronts narrow band of violent weather and the warm fronts widespread area of cloudiness and precipitation occurring in combination along the occluded front. The most violent weather occurs at the apex or tip of the occlusion. The apex is the point on the wave where the cold front and warm front meet to start the occlusion process.

Learning Objective: Describe the forma-tion, structure, and characteristics of the cold occlusion.

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