An occluded front is a common weather phenomenon in mid-latitude regions, marking a boundary between three distinct air masses. This front forms during the mature stage of a low-pressure system, often called a mid-latitude cyclone. It signifies the beginning of the cyclone’s life cycle ending, occurring when a rapidly moving cold front merges with a slower-moving warm front. The term “occluded” refers to the warm air mass being cut off from the Earth’s surface.
The Mechanism of Formation
The formation of an occluded front begins when a mid-latitude cyclone is fully developed, possessing both a trailing cold front and a leading warm front. In the Northern Hemisphere, these fronts pivot counter-clockwise around a central area of low pressure. The cold front, consisting of dense, heavy air, moves faster than the less dense warm front ahead of it.
As the cyclone intensifies, the cold front overtakes the warm front. This merger causes the intervening wedge of warmer, lighter air to be completely lifted off the ground by the surrounding denser, colder air masses. The warm air mass is thus “occluded” from the surface, creating a complex frontal boundary where the two original fronts meet.
The resulting occluded front is a composite boundary where cold air is found behind the original cold front and ahead of the original warm front, with the warm air lofted above. The point where the cold front, warm front, and occluded front converge is known as the triple point, a location associated with intense weather and the potential for a new low-pressure area to develop. This lifting process generates clouds and precipitation, as the warm, moist air cools rapidly while ascending.
Classifying Occluded Fronts
Occluded fronts are categorized into two types based on the relative temperatures of the two cold air masses involved in the merger. This distinction compares the temperature of the air mass behind the cold front with the temperature of the air mass ahead of the warm front. The resulting structure dictates how the front interacts with the surface and influences the weather pattern.
A Cold Occlusion occurs when the air mass trailing the cold front is colder and denser than the cool air mass ahead of the warm front. In this scenario, the colder, heavier air mass behind the front acts like a plow, undercutting both the warm air and the cool air ahead. This action forcefully lifts both air masses, creating a surface front that behaves like a typical cold front. Cold occlusions are often observed in the eastern parts of continents, where continental polar air is much colder than maritime air.
Conversely, a Warm Occlusion forms when the air mass behind the cold front is warmer than the cold air mass ahead of the warm front. Since the air mass ahead of the front is the densest, the slightly warmer air following the cold front cannot undercut it. Instead, the cold front rides up and over the colder air mass ahead, forcing the central warm air aloft. This occlusion often exhibits weather similar to a warm front and is more common in maritime regions near the western coasts of continents.
Observable Weather Effects
The passage of an occluded front brings a mixture of weather phenomena associated with both cold and warm fronts, resulting in a prolonged period of unsettled conditions. As the front approaches, the first visible signs are high-level cirrus clouds, which transition into mid-level altostratus as the front nears. These clouds form in the elevated warm air mass, or Trough of Warm air ALoft (TROWAL), where the warm air is forced upward and cooled.
Precipitation along an occluded front can be varied, ranging from light, steady rain or snow to heavy downpours and thunderstorms, depending on the instability of the lifted air. Cloud structures include nimbostratus, which produces sustained precipitation, and cumulonimbus clouds, which indicate intense, convective weather. A significant wind shift accompanies the front’s passage, often veering from a southerly or easterly direction to a westerly or northwesterly direction.
The temperature change at the surface depends on the type of occlusion, with cold occlusions causing a noticeable temperature drop as the coldest air moves in. After the front passes, the lifting mechanism that drives the storm dissipates, and the atmosphere becomes stable. The weather improves, with skies beginning to clear and the air becoming drier, marking the end of the storm system’s influence.