English: A cool autumn morning brought large banks of fog to southern France and northern Spain in mid-December 2021. The Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Aqua satellite captured a true-color image of the spectacular scene on December 15.

The snow-covered heights of the Pyrenees Mountains courses along the border of France (north) and Spain (south), flanked by massive banks of low cloud (fog). Both fog and snow are highly reflective, so both appear white in a true-color image. Because of the similar reflectivity and color, sometimes it’s difficult to separate the two just by looking, but there are a few characteristics that may be helpful. Snow tends to be very bright white with a generally smooth appearance, while fog leans towards a duller white color and usually looks to be finely textured. Topography is helpful as well: fog spreads across low plains, forms over rivers, and creeps into the lower valleys of hills and mountains. Snow can fall anywhere, of course, but bright white covering the high elevations is almost always snow.

The reason that fog lays low over the land is a result of how it is formed. Radiative fog develops over land when the humidity is high and the sky is initially clear, especially in the long nights of autumn. The ground, which is warmed by the sun’s ray (radiant energy) during the day, quickly loses heat in cool temperatures after the sun sets. This loss of radiant energy from the ground cools the adjacent air to below its dew point, so droplets of air form in the air right above the ground. On calm nights with little wind, this cooling effect can extend far above the surface, creating a layer of thick fog.