Stage in the development of a frontal depression during which the warm-sector air is gradually lifted from the ground and above the colder air. The word is used to describe an occluded front. An occlusion may be a ‘warm occlusion’ such that warmer air follows the frontal system, or a ‘cold occlusion’, such that colder air follows in the rear. A ‘warm occlusion’ has characteristics similar to those of a warm front, but there is an upper cold front ahead and aloft which brings showers from cumulus and cumulonimbus cloud. A ‘cold occlusion’ exhibits cold-front features, but is preceded by warm-front cloud types. Occlusions formed in the later stages in the development of travelling depressions commonly cross north-western Europe, especially the ‘warm’ type in winter and the ‘cold’ type in summer.
Subjects: Earth Sciences and Geography.