'The said Chase is mountainous and cold ground, much beclouded with thick fogges and mists.': Parliamentary Survey of Exmoor,1651
Visibility
Many parts of England, especially those relatively remote from the industrial and populous areas of both Britain and mainland Europe, enjoy good visibility. This is particularly true of much of the coastline, the mountains and the moorlands such as Exmoor. However, the mountains and moorlands are also the dullest parts of England, with annual average sunshine totals of less than 1,000 hours. Over high ground in England fog statistics are scarce but, because moist air often spreads across the country, hill fog can be both extensive and frequent and is a potential hazard to be borne in mind by walkers. Contrast the average 233 days of fog a year at Great Dun Fell in Cumbria (at 857 metres), with the six days for Carlisle (at 26 metres) in the same county. We do not know the exact figures for Exmoor because no such recordings are made.
Visibility is related to the concentrations of water droplets or solid particles, such as smoke. When water droplets are present in suitable concentration and sizes, mist or droplet fog may be formed. The occurrence of fog is usually associated with radiation cooling or advection cooling, so that air temperatures fall below dew point. Visibility means the distance at which the outlines of a building can be seen in daytime, or at night, at which ordinary house lights can be discerned. Before the First World War, the custom was to use terms such as haze, mist, or fog in referring indirectly to atmospheric visibility. Qualitative descriptions such as exceptional visibility were also used. These terms were not very precisely defined. The First World War and the years that followed saw the development of aviation that required more-precise information and this led to the development of numerical observations for visibility. In places where visibility observations are made it has been the practice to maintain a list of permanent objects. Measuring equipment such as the Gold visibility meter and the transmissometer have also been developed for measuring visibility and their use has become more common, from airfields to motorways and ports, as well as stations recording meteorological data.
Mist and fog
Fog, mist and cloud are technically the same thing. They are all formed when air cools to its dew point. At this point moisture condenses onto small particles in the air. At ground level cloud is called fog or mist depending upon the visibility. At sea or for aircraft landing and taking off purposes, a fog is defined as when the visibility is 1000 metres or less. Mist is a visibility between 1000 and 2000 metres. Over land, forecasters use the word "fog" when the visibility is 200 metres or less. This is because a car driver may be fairly happy if he can see over 200 metres while the same is not true for an aircraft pilot or sailor trying to navigate. The thickest fogs tend to occur in industrial areas where there are many pollution particles on which water droplets can grow. Hence, mist is more common than fog on Exmoor.
Air can cool in several ways. First, the air can be lifted by flowing over a hill, by convection or by the convergence of two air streams. Secondly, it can be cooled by contact with a cold ground. It can also be cooled through evaporation of moisture droplets or melting of falling snow.
Mist formed by air flowing over the Brendon Hills
Away from coasts, the most common type of fog is 'radiation fog'. It forms overnight when the ground loses heat by radiation, and cools. The ground, in turn, cools the nearby air to dew point, thus forming fog. Often the fog remains patchy and is confined to low ground. Ideal conditions for the formation of this type of fog are light winds, clear skies and long nights. Consequently, the months of November, December and January are most prone to foggy conditions, particularly in inland areas in high pressure conditions. On the southern edge of Exmoor such conditions are also common in autumn. After dawn, fog tends to disperse because it is 'burnt off' by the incoming solar radiation, some of which penetrates the fog and reaches the ground. The ground heats up, as does the layer of air near it, and fog droplets evaporate. However, in winter, fogs can be very persistent.
Radiation fog on the southern edge of Exmoor
The Exmoor coast is more affected by 'sea fog', known as 'harr' or 'fret' in other parts of Britain. This forms when moist air is cooled to saturation point by travelling over a cooler sea. The wind may then take the fog into coastal regions and is particularly known for sweeping up the valleys of the Umber, Heddon and Lyn and the Vale of Porlock. It usually dissipates when it meets the warm land surface but may come further inland at night when the land has cooled. At times it rolls along the coast with the wind and moves up and down over headlands. In still conditions it appears to come and go with the tides. This type of fog tends to occur in spring and summer, and particularly affects south western coasts. On Exmoor it is commonly seen in August. In the Bristol Channel fog is twice as common in summer as it is in winter.
Sea fog rolling westwards over the Foreland
Freezing fog is composed of supercooled water droplets (ones that remain liquid even though the temperature is below freezing-point). It is this type of fog that produces rime - the feathery ice crystals that are deposited on the windward side of surfaces such as fence posts, wires, pylons and branches. When this persists from day to day but melts in the middle of the day it forms a clear ice coating on objects and is locally known as 'ammil'.
Haze is different from mist or fog. It is a reduction in visibility due to dust or smoke in the air and is not related to cooling of moisture. It can, however, combine with fog to make smog, which is virtually unknown on Exmoor.