occlusion

A process resulting from the progressive decrease, at the level of the Earth's surface, of the warm sector of a frontal system, until its complete disappearance, as a consequence of the rapid advance of the trailing cold air mass (the one located behind the cold front). Occlusion is also called an occluded front, as it is identified with the front resulting from the merging of the cold and warm fronts of a frontal system, due to the greater speed of movement of the former. Depending on whether the trailing air mass is colder or less cold than the leading one (the one located ahead of the warm front), occlusion can be cold or warm. In a cold occlusion, the lower temperature of the trailing air means that it - being denser - penetrates beneath the leading air as it catches up with it, forcing it to rise, while in a warm occlusion precisely the opposite occurs; the colder air is the leading air, such that the trailing air, on catching up with it, rides up over it, rising. On weather maps, occlusions are usually represented with thick purple lines with pairs of semicircles and triangles of the same color attached, all pointing in the direction the occluded front is moving.