A weather front is a boundary zone where two different air masses meet, resulting in changes in atmospheric conditions, including shifts in temperature, humidity, and wind direction. An occluded front represents the final stage in the life cycle of a mid-latitude low-pressure system. It signals that the storm system is reaching maturity and beginning to dissipate.
The Mechanism of Occlusion
Occlusion begins because the cold front within a low-pressure system moves faster than the preceding warm front. This speed difference occurs because the cold air mass is denser, allowing it to wedge quickly under the lighter warm air. The faster cold front eventually catches the slower warm front, forcing the warm air mass to lift completely off the surface.
This process is categorized into two types based on the relative temperatures of the air masses. A cold occlusion occurs when the air behind the cold front is colder than the air ahead of the warm front, which is the more common scenario. A warm occlusion happens when the air behind the cold front is warmer than the air mass it is overtaking. In both cases, the central dynamic is the powerful upward forcing of the intermediate warm air.
Atmospheric Structure of an Occluded Front
The defining characteristic of an occluded front is the vertical stacking of three distinct air masses, with the warmest air lifted entirely away from the ground. When the cold front catches the warm front, the surface boundary merges, trapping a wedge of warm, moist air aloft. This trapped air is the primary engine for the resulting weather.
This lifted warm air mass forms the Trough of Warm Air Aloft, or TROWAL. The TROWAL acts as an elevated frontal system where the warm air cools and condenses as it rises. This structure, where the warm air is “occluded” or cut off from the surface, produces an extensive and layered cloud shield. Density differences maintain this structure, forcing the lighter warm air to remain suspended thousands of feet above the surface.
Cloud Types Associated with Occlusion
The clouds that form in an occluded front combine types seen in both warm and cold fronts, organized by the altitude of the lifting TROWAL. The first visible sign of an approaching occlusion often appears as high-level clouds: thin, wispy Cirrus and sheet-like Cirrostratus. These clouds are composed of ice crystals and form at the highest, leading edge of the lifted warm air wedge, far ahead of the surface front.
As the uplifted warm air mass thickens, it creates mid-level clouds like Altostratus, which appear as grayish or bluish sheets that may veil the sun or moon. Deep and prolonged lifting then leads to the formation of Nimbostratus, a multi-level, featureless gray cloud deck. Nimbostratus clouds are responsible for the steady, widespread precipitation that characterizes the passage of an occluded front.
If the cold air mass driving the occlusion is unstable, the forced uplift of the warm air can be vigorous and localized. This rapid, intense lifting generates towering vertical clouds, most notably Cumulonimbus clouds. These clouds span all three atmospheric layers, bringing intense showers, gusty winds, and the potential for thunderstorms embedded within the larger cloud shield.
Resulting Weather Patterns
The passage of an occluded front often brings a prolonged period of weather, starting with the high and mid-level cloud layers that precede the surface front. The most consistent weather effect is the steady precipitation generated by the extensive Nimbostratus layer within the TROWAL. This precipitation is moderate to heavy and can last for several hours as the broad cloud shield moves overhead.
A sharp shift in wind direction is common as the surface frontal boundary passes, often accompanied by a temporary drop in temperature. If Cumulonimbus clouds are present, the weather can become severe, including heavy downpours, hail, and lightning activity. The air mass behind the occluded structure typically brings drier conditions and a clearance of the widespread cloud cover.