Occluded fronts occur when a cold front overtakes the warm front on the same area of low pressure. Cold fronts move more swiftly than warm fronts, and since fronts rotate in the same counterclockwise direction, cold fronts are constantly in pursuit of warm fronts.
An occlusion is essentially where two things come together. To “occlude” means to close or obstruct (in dentistry for example, the occlusal of your teeth is the part where your bottom teeth meet the top, obstructing the open mouth), and in this case, it means that the warm air being drawn north by an area of low pressure is being prevented from reaching the center of circulation by the more swiftly moving cold air. With the clash of air masses becoming less significant at the heart of the system, front occlusion is typically the beginning of the end for an extra tropical cyclone.
There are two types of occlusions, a cold occlusion and a warm occlusion. The designation refers to the change in temperature before and after a front moves through a region. A cold occlusion means that after frontal passage, temperatures drop behind the front. A warm occlusion indicates the opposite.
When an occlusion passes through, the responses are similar, but more muted than if it were simply a cold or warm front passing through. Warm air floats over the occluded boundary regardless of whether it is cold or warm, a remnant of the superseded warm front. The warm air aloft dictates the difference between occluded fronts and cold or warm boundaries.
Cold occlusions produce lighter more general precipitation than true cold fronts, as air doesn’t rise as swiftly until it reaches well up into the atmosphere. The lingering warm air aloft tends to broaden the impact of the front, however, and the crisp end to precipitation behind a cold front is not seen with a cold occlusion.
Warm occlusions are actually a bit more favorable than either warm fronts or cold occlusions. The cooler air overriding the cold air is less apt to allow updrafts, and the precipitation field is reduced compared to a regular warm front. The cool air from the overtaking front will override the cold air, and subsequently force the warm air upwards, and thunderstorms are more likely with warm occlusions than with warm fronts.