In urban areas, the moisture present in fog sometimes falls from the sky in the form of snow. This is called industrial snow. The phenomenon can be observed during prolonged inversions near industrial plants that emit large amounts of water vapour, such as waste incineration plants or paper mills. In nuclear power plants with cooling towers, however, the phenomenon does not occur because the emitted steam plume is usually so warm that it penetrates the inversion layer and evaporates in the dry air.
For industrial snow to occur, two conditions must be met: The inversion must be no higher than about 900 m above sea level and the temperature at the bottom of the inversion layer must be at least -5 to -12 degrees. With these ingredients, a few centimetres of snow can fall locally, while above the thick layer of fog on the surrounding hills the sun shines down from a cloudless sky.