What situation creates a cold occlusion and what creates a warm occlusion?

Explanation:

A warm air occlusion or occluded front is less common than a cold occlusion. A warm occlusion occurs as cool air moves rapidly into an area with an existing warm front. The difference from a cold occlusion is that the approaching cool air is not as cold as the retreating cold air in the existing front.

brave new world, aldous huxley (1932)