PHENOMENON CREATES A GHOSTLY VEIL OVER THE COUNTRYSIDE IN AUTUMN
Ghost-like, a delicate drapery lies over an early morning landscape in late autumn. Mighty oaks, the silent sentinels of the fields, seem to float above its vapours, their trunks disappearing into a low-lying sea of cloud. It shrouds and swirls among the hill tops. It laps against a sweeping seashore.
This is fog, a phenomenon caused by water droplets suspended in the air, cutting visibility to less than six-tenths of a mile, or 1km. When visibility is greater, fog becomes mist. Dense fog is deemed to occur when visibility is less than 165ft (50m).
Fog readily forms in the later months of the year when there are quiet conditions under the influence of an area of high pressure. On a clear night, with little or no wind, air close to the ground rapidly loses heat to the atmosphere by radiation and steadily cools. At a certain point, known as the dew point, the air cannot hold all the moisture as vapour. Instead, minute droplets of water become visible. This is radiation fog. Sometimes called ground fog, it is a feature of low-lying meadows, filling hollows and valley bottoms.
Radiation fog is often quite localised and patchy. This is due to the diversity of landscape, with some places producing more water vapour. A slight wind can shift the fog from one place to another, quickly transforming a clear horizon into a wall of fog.
If there is little wind, radiation fog is confined to within a few feet of the ground. However, a slight increase in movement means moisture lost through condensation onto the ground’s surface is quickly replaced by moisture from above. This means the fog could thicken. If the wind turns stronger though, the fog quickly evaporates as the cold, damp surface layer is broken down.
The low winter sun can struggle to burn off radiation fog, so it may remain all day. In the extremely cold December of 1890, a complete absence of sunshine was recorded for the entire month in the Home Counties. In 1936, a fog shroud lasted nine days in the North West and Midlands.
Movement over the ground
Fog is not always restricted to quiet weather. It can result when moist, mild air is moved horizontally across a cold surface. This is advection fog, from a scientific term describing the movement of fluid. In the atmosphere, the fluid is the wind. It may occur if there has been a prolonged cold spell with a snowy or frosted landscape and then milder air comes in from the Atlantic. This mild air is very quickly chilled to its dew point and water droplets become visible. A thick mist or fog forms, even though there may be a noticeable breeze. When this type of fog is formed by a change of air mass, it is known as frontal fog.
On hills and moor
The air does not need to be completely cooled to the dew point to form upslope fogs. Instead, it condenses into fog when forced to rise by even a modest hill.
A moist airstream will often have a layer of low cloud, called stratus, that cloaks hills and mountains. This is known as hill fog. Dartmoor in Devon is a treacherous, uncompromising granite plateau when it is enveloped in one of its frequent hill fogs. The Great Dun Fell weather station in the northern Pennines, at an altitude of 2,857ft (871m), has an average of 231 foggy days a year. In Scotland, the term Scotch mist is used when low cloud gives mist or fog accompanied by a light, wetting drizzle. In many other parts of the country this is known as mizzle.
Although the term freezing fog is often used, it is misleading. When the temperature falls below freezing and fog forms, the droplets are so small, at around one to 10 microns, they do not actually freeze. The fog is made up of water droplets in a supercooled state. It is when these make contact with cold surfaces, such as fences, twigs or grass blades, that they immediately freeze.
In winter, the temperature of the air on the higher slopes of Britain’s mountains and hills is often below freezing. At the same time, they are often covered in cloud made up of supercooled water droplets. As they freeze on to surfaces, this can result in some beautiful frost accretions of ice, often on just the windward side of the object. Feathery formations of granular white crystals then accumulate that can sometimes extend many inches in length. These can create dangerous conditions. In March 1969, the 1,270ft (387m) tall Emley Moor television mast in West Yorkshire was brought crashing down by the sheer weight of this fog-created ice.
Rising and falling
When a sudden shower gives way to sunshine, whirls and wraiths of mist can be seen rising from road surfaces, roofs or pavements. Sometimes a lake or sea surface can appear to steam or smoke. This occurs when very cold air blows over a much warmer surface. The air close to the warm surface has a greater vapour pressure, leading to evaporation into the colder air above. In turn, this condenses into visible water droplets. This is sea smoke, or steam fog.
Fog that contains a high concentration of water droplets can produce what is known as fog drip when intercepted by vegetation such as tree leaves. Then, water falls rain-like under the tree and wets the ground. Gilbert White, the 18th century Hampshire naturalist, wrote: “In Newton-Lane, in October 1775, on a misty day, a particular oak in leaf dripped so fast that the cart-way stood in puddles and the ruts ran with water, though the ground in general was dusty.”
Effect on sound
A fog-cloaked landscape seems quieter, with sound muffled. This is because the air gaps between the water droplets cause higher pitched sounds to disperse their energy by bouncing between them or having to change course due to refraction. Deeper bass sounds, with a longer wave length, can penetrate a thick fog. This is why fog horns have a low timbre.
However, conditions that form radiation fog can produce what is called an inversion. In this, the temperature actually increases with height up to approximately 3,000ft (914m). The cold foggy conditions are overlain by drier warmer air. This has the effect of reflecting sounds back to the surface and extending their range horizontally.
Mist can enhance a landscape, making it surreal and dreamlike. A dense fog, however, can disorientate. It is often difficult to judge distances when the normal horizon is missing or distorted. In September 1749, it led the preacher John Wesley to offer up a prayer for deliverance from its shroud. During a visit to Cumbria, he wrote: “I kept on as I could till I came to the brow of Hartside. So thick a fog then fell, that I was quickly out of all road, but knew not which way to turn. But I knew where help could be found in either great difficulties or small. The fog vanished in a moment and I saw Gamblesby at a distance.”
Fog can be very transient, slipping away like a ghost, or last for days, shrouding the world in a coating of grey. It can soften a harsh landscape, or it can change the appearance of a much-loved one until it is almost unrecognisable. A natural phenomenon, it can roll over the countryside like a blanket. Then, as it thins, its true beauty is captured as the late autumn sun gleams through it.
Words: Ian Currie Photographs: Alamy
The feature on mist and fog appeared in the Nov / Dec 2016 issue of LandScape.
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