Fog

Fog is a suspension of tiny liquid-water droplets near the ground that reduces horizontal visibility to less than one kilometer. It forms when air becomes saturated with water vapor.

Dew point: The temperature at which air becomes saturated with water vapor at a given pressure. Fog often begins when air cools to its dew point and vapor condenses.
Valley fog: Fog that forms in a valley as dense, cool air collects near the valley floor. Sheltered terrain traps cooled air and can sustain fog after sunrise.
Visibility: The greatest distance at which an object can be seen and identified under prevailing conditions. Droplets scatter light and shorten the distances at which objects remain visible.
Mist: A suspension of tiny water droplets that reduces visibility less severely than fog. The visibility threshold separates mist from fog in standard meteorological usage.
Condensation: The change of a substance from gas to liquid. Water vapor condenses into the suspended droplets that make fog visible.
Steam fog: Fog formed when water vapor from a warmer surface condenses in colder air above it. Its wispy appearance often occurs above lakes or rivers in cold weather.
Fog signal: An audible warning, such as a horn or bell, used to mark a hazard in poor visibility. Ships and coastal hazards need audible warnings when fog obscures visual navigation.
Haze: Reduced visibility caused by fine particles or droplets dispersed in the air. Haze can resemble fog visually but often involves dry particles and broader atmospheric layers.
Radiation fog: Fog formed when the ground loses heat by radiation, cooling the air above it to saturation. Clear skies and light winds favor this common nighttime cooling pathway.
Upslope fog: Fog formed when moist air rises along sloping terrain and cools to saturation. A gentle ascent can create fog without nighttime ground cooling.

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