A weather front is a boundary separating two air masses with different properties, such as temperature and moisture. The occluded front is a composite boundary marking the mature or final stage of a mid-latitude low-pressure system (cyclone). This front forms when a faster-moving cold front merges with a preceding warm front. The process of occlusion signals that the atmospheric disturbance is nearing the end of its life cycle, having consumed the warm air that fueled its intensity.
The Air Masses Involved
The formation of an occluded front requires the interaction of three air masses at the surface. A central, relatively warm air mass is typically moist and less dense than its neighbors. This warm air mass is lifted and “cut off” from the ground during the process.
To the west (trailing the warm air) is a rapidly advancing cold air mass associated with a cold front. To the east (ahead of the warm air) is a third air mass that is older and cooler, originally displaced by the warm front. The type of occlusion is defined by the temperature contrast between the trailing cold air mass and the leading cool air mass, which determines the resulting vertical structure.
The Dynamics of Occlusion
The process of occlusion begins because the cold front moves more quickly than the warm front, causing it to catch up to the boundary ahead. As the cold front overtakes the warm front, the central warm air mass is forced to rise completely off the surface. This lifting action traps the warm air aloft between the two colder air masses meeting on the ground.
The resulting structure depends on the relative temperatures of the two cold air masses. In a cold occlusion, the trailing air mass behind the cold front is colder and denser than the cool air mass ahead of the former warm front. The very cold air plows underneath both the warm air and the less-cold air ahead of it, acting similar to a typical cold front at the surface.
Conversely, a warm occlusion occurs when the trailing cold air mass is milder than the air mass ahead of the former warm front. In this less common scenario, the milder cold air is not dense enough to undercut the leading, colder air mass. Instead, the milder air rides up and over the colder air already in place, while still lifting the central warm air mass aloft.
Associated Weather and System Dissolution
The passage of an occluded front brings a mixture of weather characteristics from both warm and cold fronts. As the warm air is forced upward, it cools, and the moisture condenses, often leading to a wide band of cloud cover and precipitation. This results in prolonged, steady precipitation similar to a warm front, sometimes with more intense showers or thunderstorms embedded within the system, especially during a cold occlusion.
Significant wind shifts are common as the front passes, and air pressure typically falls as the system approaches before rising afterward. The occlusion process is the mechanism by which the low-pressure system loses its thermodynamic engine. Once the warm air is completely lifted off the ground and isolated from the surface, the temperature contrast providing the system’s energy is removed. This causes the mid-latitude cyclone to weaken and dissipate.