Mid-latitude depressions are usually associated with warm and cold fronts separating warm and cold sectors of air. The lighter warm air rises above the heavier cold air, more gently at a warm front but more vigorously at the cold front following behind. Cold fronts usually travel faster than warm fronts, and therefore at some stage of depression development, the cold front catches up with the warm front. In cross section, the warm air is lifted right off the ground, so that the observer on the surface misses out the warm sector stage. This is known as an occlusion or occluded front.
On synoptic (weather) charts an occluded front is represented by a solid line with alternating triangles and circles pointing the direction the front is moving. On colored weather maps, an occluded front is drawn with a solid purple line.