In U. S. Weather observing practice, one of a class of atmospheric phenomena, other than the weather class of phenomena, that may reduce horizontal visibility at the earth's surface. Obstructions to vision are listed in the Manual of Surface Observations (WBAN), Circular N, as follows: fog, ground fog, blowing snow, blowing sand, blowing dust, ice fog, haze, smoke, dust and blowing spray. These are encoded as a part of an aviation weather observation.