Frost hollow (or frost pocket) is the name for low-lying area (e.g. a valley bottom or a smaller hollow) where frosts occurs more frequently than in the surrounding area. This is normally as after a dry, clear and cold night cold air drains down neighbouring slopes into a localized pocket from which it is slow(or unable) to escape. Frost hollows of larger scale (a valley or basin) are also known as cold pools. Cold pools are areas where cold air is trapped under an inversion under calm winter weather conditions.
Frost hollows are widespread along the Welsh borders or Scottish glens. A frost pocket may also occur behind a wall or hedge. In the case of the famous Rickmansworth (Herts.) frost pocket, a railway embankment prevents the natural drainage of cold air from the valley. Minimum temperatures in the pocket may be tens of degrees below the surroundings. For this reason, fruit growers try to avoid frost pockets.
Often one can actually see a frost-pocket. The accumulating layer of cold air is defined by the local topography and can be everything from a couple of inches to hundreds of feet thick. Therefore you might see a frost hollow filled with a shallow "lake" of ground fog or a cold pool in the Highlands, topped with a thick layer of stratus cloud marking the boundary of an inversion.