Fog is a meteorological phenomenon caused by a supersaturation of the air, so that it can no longer hold water vapor. The water vapor precipitates out into small droplets of condensation, or fog. The processes which form fog are similar to those which make clouds, although fog forms close to the ground, rather than higher up in the atmosphere. Because visibility can be limited in foggy conditions, care should be taken when driving or walking in the fog, especially since it tends to muffle and distort sound, potentially rendering people unaware of hazards.

Fog happens whenever the air reaches a point of extremely high humidity. Most commonly, it happens when the air rapidly cools, causing condensation to form. There are a number of types of fog, named for the conditions which create them. Drivers who are navigating in fog should use low beams or fog lights, rather than high beams, which will simply reflect from the fog and create glare.

Along the shores of oceans and large bodies of water, advection fog happens when moist air from the water passes over the cooler surface of the land. Frequently, warmer weather inland sucks the moist air across the land, creating a thick blanket of fog. Advection fog happens most frequently around the ocean because the salt increases the humidity, and condensation can form at a much lower humidity level around salt.

Another common type of fog is radiation fog. Radiation fog usually occurs after dark, when the Earth radiates heat outwards. As the heat rises, it is cooled, causing saturation conditions and fog. Radiation fog usually clings close to the ground, and disappears by mid-morning, once the day warms up enough to dissipate it. A variation on radiation fog, tule fog, is found in the Central Valley of California. Tule fog happens when cold mountain air sinks into a depression such as the valley at night. Warmer air above it presses the cold air down, causing fog to form and linger for days. This type of fog occurs in the fall and winter, when conditions on the mountains are colder.

A rare type of fog called ice fog can only be formed in extremely cold conditions such as those at the Arctic and Antarctic. When the ambient air temperature is substantially below the freezing point, but still humid, the water droplets which would normally form fog turn into small ice crystals instead. Ice fog can be dangerous to drive in, as the crystals cling to windshields and headlights.