Valley Fog
Valley fog is another special case of radiation fog. The formation process also relies on the land surface cooling by emitting radiation to space, allowing the air to reach its dew point temperature and causing moisture to condense.
Most people know that warm air rises. This is because warm air is less dense than cool air – it weighs less, and therefore becomes buoyant and rises above the cool air. Equally, cold air sinks. In hilly areas, on a calm and cloudless night, the land surface on the hills cools. This in turn cools the air directly above it. The cooler air then sinks down into the valleys, much like water flowing through a river. Pooling of cold air means that fog can form more quickly, and persist for longer, in valleys than over flat terrain. At times in the Winter, some valleys or basins (for example, the Mackenzie Basin) will see fog that persists for days, and does not even clear during the height of the day. This is because the weak sunshine is not enough to warm the extensive pool of cold air sitting in the valley.
Valley fog, Dovedale River, Tasman District. Photographed by Sheryl Waters.
Valley fog in the Wakatipu Basin, photographed by Marin Corinthian Kohn.
