Fog

Fog is a visible aerosol of tiny water droplets or ice crystals suspended in the air close to the Earth's surface, in effect a cloud that has formed at ground level, and it is shaped by nearby water, terrain, and wind. It forms when the temperature of the air comes within about 2.5 degrees Celsius of its dew point and water vapor condenses onto tiny particles of dust, ice, or salt, a process that can be triggered by air rising, rain falling into drier air below, evaporation from the ground or water, plant transpiration, or air passing over a warmer or cooler surface. Meteorologists distinguish several types by how they form: radiation fog develops after sunset as the ground radiates away its heat under clear, calm skies; advection fog forms when moist air blows across a cooler surface, often over cold ocean water; sea fog needs tiny salt crystals from sea spray to act as condensation nuclei; and freezing fog deposits rime ice as its droplets freeze on contact with surfaces.