- 1a : vapor condensed to fine particles of water suspended in the lower atmosphere that differs from cloud only in being near the ground
- b : a fine spray or a foam for firefighting
- 2: a murky condition of the atmosphere or a substance causing it
- 3a : a state of confusion or bewilderment <spent the morning in a fog>
- b : something that confuses or obscures <hid behind a fog of rhetoric>
- 4: cloudiness or partial opacity in a developed photographic image caused by chemical action or stray radiation
Fog is a collection of water droplets or ice crystals suspended in the air at or near the Earth's surface. While fog is a type of a cloud, the term "fog" is typically distinguished from the more generic term "cloud" in that fog is low-lying, and the moisture in the fog is often generated locally (such as from a nearby body of water, like a lake or the ocean, or from nearby moist ground or marshes).
Fog is distinguished from mist only by its density, as expressed in the resulting decrease in visibility: Fog reduces visibility to less than 1 km, whereas mist reduces visibility to no less than 1 km but less than 2 km. For aviation purposes in the UK, a visibility of less than 2 km but greater than 999 m is considered to be mist if the relative humidity is 95% or greater - below 95% haze is reported.
The foggiest place in the world is the Grand Banks off the island of Newfoundland, the meeting place of the cold Labrador Current from the north and the much warmer Gulf Stream from the south. Some of the foggiest land areas in the world include Argentina, Newfoundland, Labrador and Point Reyes, California, each with over 200 foggy days per year. Even in generally warmer southern Europe, thick fog and localized fog is often found in lowlands and valleys, such as the lower part of the Po Valley and the Arno and Tiber valleys, as well as on the Swiss plateau, especially in the Seeland area, in late autumn and winter.Other notably foggy areas include coastal Chile (in the south), coastal Namibia, and the Severnaya Zemlya islands.