An occluded front represents a type of weather boundary where different air masses interact, leading to distinct atmospheric conditions. It forms as a faster-moving cold air mass overtakes a preceding warm air mass, causing the warm air to lift from the surface. This interaction between three air masses—a cold, a cool, and a warm—results in a specific sequence of weather changes.
How Occluded Fronts Form
Occluded fronts develop within mature low-pressure systems when a faster cold front converges with a slower warm front. The cold front, denser, catches up to the warm front, which is already pushing into a cooler air mass. As the cold air mass advances, it wedges underneath the less dense warm air, forcing that warm air to rise from the ground. This process “occludes” or cuts off the warm air from the surface.
The interaction involves three air masses: the cold air behind the cold front, the warm air, and the cool air ahead of the warm front. Their interaction determines the type of occlusion. In a cold occlusion, the air behind the cold front is colder than the cool air ahead of the warm front, causing it to plow underneath both. Conversely, a warm occlusion occurs when the air behind the cold front is warmer than the cool air ahead, leading the cold front to ride up over the cool air.
The Weather During an Occluded Front’s Passage
As an occluded front approaches, cloud development begins with high-level cirrus, followed by mid-level altostratus. These gradually thicken into nimbostratus clouds, dark and dense enough to obscure the sun. If the lifted warm air is unstable, cumuliform clouds may also develop, sometimes leading to embedded thunderstorms. These clouds significantly reduce visibility.
Precipitation associated with an occluded front is prolonged and steady, often as rain or snow. Intensity varies from light to heavy, depending on the moisture and stability of the lifted warm air. More intense precipitation, including heavy rain, thunderstorms, or hail, can occur, particularly with cold occlusions where uplift is more vigorous.
Temperature changes during the passage of an occluded front are complex and not always straightforward. There might be an initial slight drop in temperature as cooler air arrives, followed by brief stabilization or even a slight rise as the warm air aloft passes. A more significant temperature drop occurs as the colder air mass behind the front dominates. Wind direction shifts from a southeast or south flow to a southwest or west. Barometric pressure usually falls as the front approaches, reaching its lowest point during passage, then begins to rise once the system has passed.
After the Occluded Front Passes
Once an occluded front passes, the weather typically stabilizes. Widespread precipitation usually ceases, and skies gradually clear. The dominant air mass following the front is colder, leading to a significant drop in surface temperatures.
Wind direction shifts to a northerly or westerly flow. With the departure of cloud cover and cessation of precipitation, visibility improves. While stormy conditions subside, scattered showers or squalls can still occur if the air mass behind the front remains unstable.