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Occluded Front Basics
Developing cyclones, known as low pressure systems, typically have a preceding warm front, located to the east of the low pressure center. Behind this warm front, they have a faster-moving cold front, located to the south of the pressure center. These fronts represent the boundaries between three air masses, or sectors: warm air, cool air and colder air. When the cold front catches up and overtakes the warm front, an occluded front is formed. The warm air mass is forced over both of the colder air masses. Occluded fronts begin at the pressure center and spread outward along the original cold front.
Cold Occluded Fronts
Cold occluded fronts, the most common in North America, occur where the new air behind the cold front is colder than the existing cool air ahead of the warm front. This type of occluded front behaves much like a traditional cold front. As a cold occluded front advances, both the warm air and cool air are forced upward as the colder air plows underneath.
Warm Occluded Fronts
Warm occluded fronts occur where the new air behind the cold front is milder than the existing cold air ahead of the warm front. This type of occluded front behaves much like a traditional warm front. As a warm occluded front advances, it sandwiches between the warm air and colder air. Both the warm air and the milder air are forced upward over the existing layer of cold air.
Cold Occluded Front Effects
Cirrus clouds will form in advance of a cold occluded front, transitioning to stratus and nimbostratus clouds as it passes. The potential also exists for the formation of scattered cumulus clouds. Cold occluded fronts typically produce continuous, heavy precipitation. Temperatures will remain cool in advance of the cold occluded front, dropping as the front passes.
Warm Occluded Front Effects
Cloud formation associated with a warm occluded front will be similar to a cold occluded front. However, generally more horizontal development than vertical development will occur, limiting cumulus formation and extending the range of cloud cover. Warm occluded fronts typically produce continuous, heavy precipitation. Unlike with a cold occluded front, temperatures will be cold in advance of a warm occluded front, becoming milder as the front passes.
Variables Impacting Occluded Front Effects
Cloud formation is always dependent on atmospheric moisture and the gradient of uplift. Cool air is always more dry than warm air. A rising layer of cool air, as occurs with an occluded front, has less moisture available for cloud formation and precipitation. This effect increases as you move closer to the center of low pressure and away from the cold front, where the warmer, moist air is being sharply uplifted.