An occluded front develops when a cold front overtakes a warm front, lifting the warm air mass away from the ground. This creates a unique boundary where different air masses converge, leading to distinct weather patterns.
Formation of Occluded Fronts
The formation of an occluded front begins within a low-pressure system when a faster-moving cold front catches up to a slower-moving warm front. The cold front’s denser, colder air mass undercuts the less dense warm air, forcing it to rise and separate from the surface.
At the point where the cold front merges with the warm front, a “triple point” forms, signifying the convergence of three distinct air masses. The original warm air mass becomes elevated above the surface, situated between the colder air from the original cold front and the cooler air ahead of the warm front. This complex vertical structure is characteristic of an occluded front.
Associated Weather Phenomena
Occluded fronts bring widespread cloudiness and precipitation as warm air is forced upward, cools, and condenses. Skies become overcast with layered clouds such as stratus, nimbostratus, and altostratus, sometimes containing embedded cumuliform clouds. High clouds like cirrus and cirrostratus may precede the front’s arrival.
Precipitation is common, ranging from light to moderate rain, snow, or mixed precipitation. If the lifted warm air is unstable, thunderstorms can develop within the system. Visibility can decrease due to the mix of cloud types and ongoing precipitation.
Temperature shifts occur as air masses interact, though surface changes may be less pronounced since the warmest air is aloft. A gradual temperature drop is often observed. Wind directions typically shift, and strong winds can accompany the front’s passage.
Cold and Warm Occlusions
Occluded fronts are categorized into two main types based on the relative temperatures of the air masses involved.
Cold Occlusion
A cold occlusion occurs when the air mass behind the advancing cold front is colder than the cool air mass ahead of the warm front. The very cold air undercuts both the warm air and the cooler air, forcing them to rise steeply. This type often brings more intense weather, including heavy rain and potentially severe thunderstorms, with characteristics similar to a cold front passage.
Warm Occlusion
A warm occlusion forms when the air mass behind the cold front is milder than the colder air mass ahead of the original warm front. The milder cold air rides up and over the colder air ahead, while still lifting the warm air mass aloft. Weather associated with warm occlusions typically includes lighter precipitation and extensive overcast conditions, often resembling the more gradual changes seen with a warm front. If the lifted air is sufficiently unstable, even warm occlusions can produce more severe weather.