File:Low clouds in central Europe (afternoon overpass) (MODIS 2015-11-12).jpg

English: Several large banks of low clouds (fog) stretched over central Europe in early November 2015. The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA�s Terra satellite captured this true-color image on November 1 as it passed over the region.

Fog fills valleys in Switzerland, France, Germany and Luxembourg while the high peaks of the Alps rise above the cream-colored cloud. Another bank of fog overlies the coast of the English Channel and low clouds have enveloped parts of the United Kingdom, including London.

Fog is simply clouds which form at ground level instead of high in the atmosphere. Like all clouds, fog forms when the local air reaches its dew point, which is the temperature at which water vapor condenses into tiny droplets. Usually, sunlight warms the air enough that fog burns off by mid-morning, but as autumn advances, nights lengthen, and temperatures drop persistent fog in the valleys in this region become more common. Wind also helps dissipate fog banks, but if no wind is present, valley fog can persist for days at a time.