In a mature mid-latitude cyclone the cold front often travels faster than the warm front. When the cold front catches up and overtakes the warm front, the warm air is lifted entirely off the surface. The resulting boundary is an occluded front.

Occlusions can be cold-type or warm-type depending on the relative temperatures of the air masses involved. Clouds and precipitation are common along the occluded front, though the intensity is usually less than along the original cold front. The surface temperature contrast is weaker because the warm air is no longer at the ground.

On weather maps occluded fronts are marked with a purple line combining triangles and semicircles. Their appearance signals that the cyclone is past its most intense stage and is beginning to weaken. Understanding occlusion helps forecasters anticipate the evolution of storm systems.

Satellite imagery often shows the wrapped-up cloud pattern associated with occlusions.