The visible haze of water droplets near the Earth's surface is called fog. It forms when water vapor condenses into tiny liquid droplets—or, in very cold conditions, ice crystals—that remain suspended close to the ground. Meteorologists generally distinguish fog from a low cloud by its position and its effect on visibility.
Fog can develop in several ways. Radiation fog forms when the ground cools overnight and chills the air above it; advection fog appears when moist air moves over a colder surface; and sea fog develops near oceans when humid air cools over cold water. Hills, valleys, lakes, and marshes can all help create it.
A common mix-up is mist. Mist is essentially a thinner, less visibility-reducing suspension of droplets, while fog is conventionally associated with visibility below 1 kilometer. Smog is different again: it is polluted air, not simply condensed water. Fog has also played a major role in transportation history, contributing to ship collisions, aviation hazards, and road accidents.