Fog is a sparsely dense not very thick layer of cloud that forms on contact with the ground in particular meteorological conditions. Generally the drops of water in a fog are smaller and less numerous than in a cloud, for this reason fogs are less dense and more transparent than clouds.
Fogs form when there is a difference in temperature between the ground and the air above it. Radiation fog forms after sunset, when the temperature of the ground decreases, provoking a slow cooling also of the air above. If the temperature drops below dew point, a fog forms. Generally the formation of fogs is favoured by the presence of stretches of water that provide water vapour to the air, thus making it more humid.
Advection fog, instead, forms when the wind brings humid warm air over colder stretches of land, or on the contrary, when cold air rests above a warmer stretch of water, such as the fogs that form over the North Sea , where warm humid air, that moves following the Gulf Stream, comes into contact with the cold waters of the North Sea, or along the coasts of Peru and Ecuador, where, on the contrary, the fog forms due to the contact of the cold waters of the Humboldt current and the hot humid equatorial air. With a similar mechanism also coast (sea) fogs form, these can be seen in the morning along the coasts and dissolve rapidly during the course of the morning.
Persistent fogs, that are typical in the winter season in the Po valley, instead, are due to cold air that descends into an anticyclone area: if there is humid air near the ground, due to the presence of sheets of water or due to the humidity in the soil (for example the humid soil in the paddy fields), the humidity in the atmosphere condenses giving rise to heavy fog that persists as long as the high pressure conditions continue.

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