A cold occlusion occurs when a cold front lifts a warm front. This can result in the piling up of three air masses on top of each other. In the case of a cold front and a warm front, there are typically two air masses that are interacting. On the diagram below, just behind the cold front there are shown to be two air masses. The colder dense air at the surface and a different air mass aloft. This can depend on the depth of the cold front. Some cold fronts are shallow while others are very deep as they pass over the surface. The cold front in the diagram below is more of the shallow variety. Warmer air lifts over the dense colder as the cold front moves through.

Position 2 on the diagram is just ahead of the warm front. At this position there is relatively cool air at the surface while there is warm air advecting over the top of the denser relatively cool air. The occluded front is what lifts up both of these air masses. The three air masses stacked on top of each other occur just behind the occluded front. The two air masses at position 2 are lifted up by the third air mass of cold air behind the occluded front. With all this lifting taking place, when moisture is in place, significant precipitation and storms can occur. Over time though, the occluded front grows in length since it is a relatively fast moving front and starves the center of the low pressure from heat and moisture and this typically causes a weakening of the mid-latitude cyclone.