When a cloud forms at ground level and reduces visibility to less than one kilometer, it is called fog. Tiny water droplets or ice crystals suspend in the air, scattering light and creating a white or gray veil.

Radiation fog develops on clear, calm nights when the ground cools rapidly. Advection fog forms when moist air moves over a colder surface, common along coasts. Upslope fog appears when air is forced to rise and cool over rising terrain. Steam fog can rise from warm water into cold air.

Fog complicates driving, aviation, and shipping. It usually lifts as the sun warms the surface or as wind increases. Freezing fog deposits rime ice on surfaces when temperatures are below freezing. Dense fog advisories alert the public when visibility drops to dangerous levels.

The same physics that create clouds aloft produce fog at the surface.