This post builds on two previous posts: Introduction to Air Masses and Warm and Cold Fronts Explained.

In an occluded front, a cold front catches up to a warm front. There are two types of occluded fronts, cold-occluded and warm occluded. The difference between them lies in whether the air behind the cold front is warmer or colder than the air in front of the warm front. As we will see below, in a cold-occluded front, the air behind the cold front is colder than that in front of the warm front.

As the cold front approaches the warm front, it’s colder air not only pushes the warm air upward as in a regular cold front, but it also pushes up the cold air in front of the warm front. This is because it is more dense (colder). The moment the cold front overtakes the warm front is the initial occlusion, as shown in the animation below in the second of three slides. The first slide shows the cold front as it is about to overtake the warm front, and the third slide shows the actual cold-occlusion. All slides have typical cloud and precipitation patterns.

Before a cold-occluded front passes the winds will generally be from the south or east, and they will turn to west-northwest after passage. The temperature will go from cold to very cold, with the pressure falling before passage, reaching a minimum while the occlusion is passing and rise afterward. Clouds found are typical for warm and cold fronts, only combined. Before passage, expect cirrus and stratiform clouds, changing to nimbostratus or cumulonimbus during passage, and changing to various middle-to-high stratiform clouds with a possibility of scattered cumulus after passage.

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