Definition of fog: a suspension of very small, usually microscopic water droplets in the air, reducing visibility at Earth's surface. The reduction in visibility depends on the structure of fog, especially on the number of droplets per unit volume and on the size of distribution of the droplets. This structure is determined mainly by the nature of the atmospheric aerosol, the mode of formation and its age. It may vary a great deal in space and time. When the air is polluted, it is referred as smog. The terms fog and mist are used to indicate the different intensities of the phenomenon: "mist" being synonymous with light fog with visibilities equal or greater than 1 km, and "fog" with visibilities less than 1 km. Fog is in fact nothing more than a "cloud" on Earth's surface.

The picture was taken on a wintry day with visibilities less than 1 km, and the sky was mostly covered by Stratus nebulosus opacus, sometimes Stratus nebulosus translucidus.