cyclolysis, in meteorology, the process by which a cyclone weakens and deteriorates. The decay of an extratropical cyclone results when the cold air, from the north in the Northern Hemisphere or from the south in the Southern Hemisphere, on the western side of such a cyclone sweeps under all of the warm tropical air of the system so that the entire cyclone is composed of the cold air mass. This action is also known as occlusion.