Fog is essentially a cloud that has descended to ground level. Meteorologists typically define it as any low-lying cloud that cuts visibility below 1 kilometer, or roughly 0.62 miles.

The term covers more than just suspended water droplets. Fog can also consist of smoke particles, ice crystals, or combinations of the two.

Fog forms through condensation, when water vapor in the atmosphere turns into tiny droplets that remain suspended in the air. This typically occurs when the air temperature and the dew point converge closely enough.

Fog and mist are related but not interchangeable. Fog is the denser of the two and impairs visibility far more significantly, while mist is lighter and more transparent.

Radiation fog tends to develop on still, clear nights. Without cloud cover to trap heat, the ground cools rapidly after dark, chilling the air above it until moisture begins to condense.

Advection fog occurs when warm, humid air drifts across a cooler surface. A well-known example can be found off Newfoundland, where air carried by the warm Gulf Stream passes over the frigid Labrador Current.

Steam fog has the visual quality of smoke rising from open water. It develops when a mass of cold air passes over comparatively warm water, triggering quick evaporation followed by condensation.

In extremely cold climates, water droplets in fog can freeze into suspended ice crystals, producing ice fog. The resulting visibility can be even more limited than ordinary fog.

San Francisco has a worldwide reputation for fog, a product of cool air arriving off the Pacific and colliding with warmer temperatures inland.

Fog does not always require full saturation of the air. When pollution levels are high, the particles present can act as condensation nuclei, allowing fog to form even at relative humidity below 100 percent.

Valley terrain is particularly susceptible to fog because cold, dense air naturally drains downhill and pools overnight, concentrating moisture near the surface.

Tule fog is a thick variety of radiation fog that settles over California’s Central Valley during the winter months, sometimes reducing visibility to near zero.

Fog has occasionally shaped the outcome of military engagements. A dense fog in August 1776 allowed George Washington to withdraw his forces from Brooklyn under British noses during the Battle of Long Island.

For pilots, fog ranks among the most dangerous weather conditions, capable of making even a familiar runway effectively invisible.

During the Second World War, Britain developed a system known as FIDO, short for Fog Investigation and Dispersal Operation, which used lines of burning fuel alongside runways to burn off fog and allow aircraft to land safely.

In arid places such as Chile’s Atacama Desert, purpose-built mesh nets are used to harvest moisture from passing fog, converting it into a source of fresh water.

Artificial fog, produced by combining water with glycol or glycerin, is a standard tool in live entertainment and filmmaking, appearing regularly in concert productions, haunted attractions, and movie sets.