A weather front is the boundary where two air masses with different properties, such as temperature and humidity, meet. While most common fronts—cold, warm, and stationary—involve two distinct air masses, one specific boundary incorporates three. The only frontal structure that involves three separate air masses simultaneously is the Occluded Front. This complex atmospheric feature typically forms during the late stages of a mid-latitude low-pressure system’s life cycle.
Identifying the Occluded Front
An occluded front is a composite boundary created when a faster-moving cold front overtakes a slower-moving warm front. This action forces the intervening warm air mass to be lifted completely away from the Earth’s surface. The structure involves three distinct air masses in a vertical alignment: the warm air mass lifted aloft, the cold air mass advancing from behind, and a cooler air mass originally ahead of the warm front. This vertical layering sets the occluded front apart from other surface frontal systems.
The lowest layer consists of two cold air masses meeting at the surface, with the third, warmest air mass resting above them. The leading cold air mass is situated ahead of the original warm front boundary. The trailing cold air mass is the one that has caught up, pushing the system forward. The central, warm air mass is no longer in contact with the ground but is “occluded,” or cut off, from the surface by the meeting of the two colder air masses beneath it.
The Mechanics of Occlusion Formation
The process begins in a developing mid-latitude cyclone, which rotates counter-clockwise in the Northern Hemisphere, drawing warm air northward and cold air southward. As the system matures, the cold front, which is denser and moves faster, begins to rapidly advance on the warm front. The point where the cold front first merges with the warm front is known as the “triple point.”
From this triple point, the cold front continues to sweep around the low-pressure center, merging with the warm front. This action is driven by cold air being denser than warm air, allowing the cold front to rapidly undercut the less-dense warm air. The warm sector of the storm, which was once at the surface, is progressively squeezed upward into the atmosphere.
This lifting action defines occlusion, resulting in the warm air mass being entirely separated from its source of moisture and heat at the ground. The surface frontal boundary then becomes the line where the two cold air masses—the trailing and the leading—meet. This mechanical lifting of the warm, moist air provides the energy and instability for the weather associated with the mature storm system.
Distinguishing Between Cold and Warm Occlusions
The occluded front’s structure is determined by the relative temperatures of the two surface air masses, leading to two primary types. The relative coldness between the air mass trailing the cold front and the air mass ahead of the original warm front dictates how the lower two air masses interact. This hinges on which cold air mass is denser and therefore able to undercut the other.
A Cold Occlusion forms when the cold air mass advancing from behind the original cold front is colder and denser than the cooler air mass ahead of the warm front. In this scenario, the newly arriving, colder air mass plows under both the original warm air and the less-cold air mass ahead. The resulting surface boundary acts much like a cold front, with the coldest air advancing at the surface.
In contrast, a Warm Occlusion occurs when the cold air mass trailing the cold front is warmer, or less cold, than the very cold air mass already situated ahead of the warm front. Because the trailing air is less dense, it cannot undercut the colder air mass at the surface. Instead, the trailing cold front rides up and over the entrenched, colder air mass. The resulting vertical structure and weather patterns often resemble those of a warm front.
Weather Patterns Associated with Occlusions
The weather produced by an occluded front is often a complex combination of cold and warm front characteristics, sometimes creating the most intense weather of the low-pressure system. The forced ascent of the warm, moist air mass high into the atmosphere leads to atmospheric instability and extensive cloud formation. This uplift creates a broad mixture of cloud types, including high-level cirrus clouds, mid-level altostratus, and low-level nimbostratus clouds.
Precipitation tends to be widespread and prolonged, starting with lighter, continuous rain or snow associated with the initial lifting, similar to a warm front. As the core of the lifted air mass passes overhead, the weather can transition to heavy showers, squalls, or thunderstorms, characteristic of a cold front. This intense precipitation occurs in the Trough of Warm Air Aloft (TROWAL), where the maximum moisture is being lifted and condensed. After the system passes, the wind shifts, and the air mass becomes drier and colder, indicating the storm system is weakening.