An occluded front (or occlusion) forms when a cold front catches up with a warm front in a maturing depression, lifting the warm air entirely off the surface. This typically occurs 24–48 hours after the depression forms, as the faster-moving cold front overtakes the slower warm front. On weather maps, occluded fronts are depicted as purple lines with alternating triangles and semicircles.
Two subtypes exist: a cold occlusion (the air behind the cold front is colder than the air ahead of the warm front — more common in winter), which behaves like a cold front; and a warm occlusion (the air behind is less cold than the air ahead — more common in maritime areas), which behaves like a warm front. In practice, most occlusions exhibit mixed characteristics, with a complex band of precipitation including both steady rain and embedded showers.
The occlusion process marks the beginning of the decay phase of a depression: by lifting the warm sector off the ground, the storm loses its main energy source (the temperature contrast across the fronts). However, occluded fronts can still produce significant weather — prolonged rain, hill snow, and moderate winds — especially where they wrap around the depression's centre. In the Iberian Peninsula, occluded fronts from Atlantic depressions frequently bring rain to northern and western Spain, though the precipitation is generally less intense than at well-defined cold or warm fronts. Understanding occlusions is key to interpreting the later stages of cyclone evolution on weather maps.