Fog and Mist

It was on the ninth of November [...] He was walking home about eleven o'clock from Lord Henry's, where he had been dining, and was wrapped in heavy furs, as the night was cold and foggy.

The beginning of chapter twelve of Oscar Wilde's The Picture of Dorian Grey describes many things about fog and how we feel in it. First, fog is very common in fall and winter. Also fog is often forming during night-time and the calm and cold weather together with reduced visibiilty and the damp, close air is evocing a feeling of loneliness and fear.

Meteorologically speaking fog is nothing but a very low stratus (latin: layer) cloud. In fact this cloud often even touches ground. Fog is made of tiny liquid water droplets, lightweighted enough to remain suspended in the air. Their average diameter is varying between 0.01mm and 0.1mm and causing any light present to be reflected off in different directions. This is the cause of the whitish vale that we all know. The term fog is used when visibility reduces to less than 1km and the relative humidity exceeds 95%.

Whereas mist is reported when visibility exceeds 1km. Obscuration by dry particles is defined as haze , which must not be mixed up with the popular, but somewhat vague term 'hazy skies' describing high thin clouds (often cirrus) obscuring the sun. Another form of fog is sea, lake or river smoke.

The main criteria for fog formation is water-saturated air. When air is saturated, the water vapor turns into liquid water droplets in a process called condensation. The two ways air can become saturated are cooling the air to its dewpoint temperature or evaporating moisture into the air and increasing its water vapor content.

The five most important factors for forecasting fog formation are 1) long nights during colder months; 2) clear skies and light winds; 3) moist air; 4) a low-level temperature inversion and the 5) sufficient supply of condensation nuclei.

the most common cause of fog is cooling by radiation, when 1) the ground surface radiates heat and cools; 2) the layer of airnext to the surface is cooled by contact with the surface. 3) As fog forms itradiates heat (condensation heat) and thus becomes cooler. Eventually 4 ) gentle movement of air increases mixing and the fog will gradually become thicker reducing visibilities to zero.. Fog usually clears soon after dawn as the sun heats up the air again, allowing it to hold more moisture. The fog is "burned off".

Types of fog are:

Radiation or ground fog - in mountain areas and cold pools often seen as valley fog. Radiation fog might be rather shallow over damp and wet ground after a rainy night and is sometimes called precipitation fog. Radiation fog might also form as a low stratus cloud forming just underneath an inversion, gradually expanding and descending towards the ground surface.

Advection fog - is also called sea fog, when warm air flows over relatively cold sea surface, but is just as common over land. Advection fog is often associated with the passage of cold and warm fronts and thus is also known as frontal fog.

Sea smoke or river smoke. Mainly caused by evaporation. This type of fog is rather shallow and lies generally below 10m in height from the surface and is locally restricted to places with large temperature differences with the air colder than the water.

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