Officially, fog is defined as visibility of less than 1,000 meters. This limit is suitable for aviation purposes but for people on the roads an upper limit of 200 meters is more realistic.
Severe disruption to road, rail and air traffic occurs when the visibility falls below 50 meters.
What is fog made up of?
Just like clouds, even fog is made up of tiny water droplets suspended in the air. We can generally see dense fog in and around industrial areas where there are many pollutants on which water droplets can grow.
Types of fog
Fog which is composed entirely or mainly of water droplets is generally classified according to their process of formation. Following are the main types of fogs:
Clear skies and calm conditions during the winter season generally lead to radiation fog. The land gets cooled overnight by thermal radiation, which in turn cools the air close to the surface as well. This reduces the ability of the air to hold moisture, leading to condensation and occurrence of fog. Radiation fog gradually dissipates after sunrise as the ground warms up.
When moist air passes over a cool surface, it gets cooled considerably. This gives rise to advection fog, which is very common at sea when moist tropical air moves over cooler waters. This kind of sea fog can get transported over the coastal areas. Advection fog also appears when a warm front passes over an area with snow cover.
As and when cold dense air settles into the lower parts of a valley, it condenses and forms fog. This is known as valley fog. It is the result of a temperature inversion and is confined to the local topography. Such fog can last for several days in calm conditions during the winter season.
Upslope fog or hill fog forms when winds blow up a slope, which is also known as the orographic uplift. As the air rises, it gets cooled down, allowing moisture in it to condense.
Freezing fog is made up of super cooled water droplets - which remain in the liquid form even when the temperature is below freezing point. In this type of foggy weather, rime (composed of feathery ice crystals) gets deposited on the windward side of vertical surfaces such as lamp posts, overhead wires, pylons, fence posts, transmitting masts.
Evaporation fog is formed when cold air passes over a warmer surface – could be water or moist land. Sometimes this type of fog often produces freezing fog. When this relatively warm water evaporates into the lower layers of air, it warms the air. This causes the warmed up air to rise and mix with the cooler air that has passed over the surface. The meeting of the moist warm air with the colder air allows condensation and fog to occur.
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