There are four main types of fog and they are in essence cloud that has formed at the earth's surface. Three types of fog result with air being cooled to the point where the air no longer holds the moisture in vapour form therefore condensing into water droplets. A fourth type comes about by air remaining at the same temperature but accumulating more water until reaching saturation point, when water condensation occurs. Knowing the types and causes of fog helps to predict its duration and extent.
The amount of moisture or water vapour a given amount of air can contain depends on its temperature. Warm air holds more water vapour than cold air. Relative humidity is the ratio of the actual amount of water vapour in the air, at a certain temperature, to the maximum amount of water vapour that it could hold at that temperature. The relative humidity is expressed as a percentage; a relative humidity of 100 percent is air containing the maximum amount of water possible at a given temperature and termed as saturated.
When warm, moisture-laden air cools, it increase relative humidity and when cooled sufficiently it reaches saturation. [ Dew point ] temperature is the temperature at which this happens, as the moisture begins to condense out of the air forming water droplets, creating dew, clouds or fog.
[ Radiation fog ] often called land fog, forms over land at night in clear conditions. With the passing of night, the land radiates its heat upwards and with no clouds trapping the heat, the air in contact with the land cools. When earth's surface temperature falls below the dew point of the adjacent air, it causes it to become saturated with water vapour condensing to produce fog.
Radiation fog only forms when the land cools rapidly and under a warm, moist airstream that usually comes off the sea. Radiation fog requires low wind speeds; otherwise it mixes with the cold air and warms the surface air. Commonly formed under high-pressure systems that bring settled weather and clear skies, radiation fog forms as mist in low valleys, gradually thickening and deepening as more air is cooled.
In the early morning, it may extend a distance out to sea dispersing if the water temperature is higher than the land. Radiation fog generally poses no real hazard to sailors, except in rivers and estuaries, persisting for some hours. Radiation fog disperses quickly as the sun rises with the land heating up, warming the air, raising its dew point, and then lifting the fog. If at dawn overcast conditions prevail, the land will take longer heating up with the fog persisting.
If the ground is damp or the humidity already high under these conditions, continued cooling results to and beyond the dew point, causing condensation in the form of dew and mist. When this air cools further, it produces dense fog in layers to hundreds of feet above ground level. In valleys, mist and fog roll down from the hillsides and form cold 'lakes' of air capped by warmer air. In winter, hoar frost forms when the dew point is below 0°C (32°F).
Seasonal differences have different effects with the rising sun in summer burning the fog off or causing it to rise as dispersing stratus cloud. Additional moisture in the air in spring and autumn coupled with the sun rising later means it takes time for the fog to disperse. Offshore, the only obvious effect of these conditions will be that they hold back the formation of sea breezes in spring and autumn weather.
In winter, especially when there are settled conditions of high pressure radiation fog, driven by vehicle exhaust gases and industrial pollution, the fog may continue for days.
[ Advection fog ] also called sea fog, is normally found at sea and caused by a warm, moist airstream blowing over cooler water. This occurs principally in spring, when the sea is cold and the rising sun is warming the moisture saturated winter land to produce saturated air. Common during winter and spring, it also may occur in summer. Areas of sea turbulence bringing cold water currents to the surface also produce advection fog.
Warm, moist airstreams moving from temperate to polar latitudes form large banks of sea fog along wide fronts and are gradually cooled by moving over cold water. Advection fog is the greatest hazard to sailors as it is very thick and persistent, even in the presence of a strong wind. With advection sea fog, expect it to be accompanied by winds of up to 12 knots to 20 knots if the incoming air is already at or near dew point.
When it occurs, it is important to determine the boat's accurate position. When onshore winds meet cliffs or mountains often the fog is lifted for up to a mile offshore, generating a corridor of visibility along the shore. Advection fog disperses when a wind change brings drier air.
[ Frontal fog ] arises along the warm front of a depression when warm air rises over cold air cooling rapidly to below its dew point forming a long narrow strip of fog along the front. Frontal fog is seen as low cloud, which falls to sea level and also develops as high-level fog above clear conditions, obscuring high shore features from view. Frontal fog is not for the most part persistent, but causes problems with pilotage if navigating using landmarks such as transits or lighthouses.
Warm moist air trapped between two colder fronts may form warm sector mists in the area of a depression. As occlusion takes place, the compressed warm air becomes saturated, with only a small drop in temperature as it contacts a cold sea, or air producing fog.
[ Arctic sea smoke ] found in arctic and polar regions, occurs when cold air absorbs moisture as it passes over a warmer sea. The excess moisture unable to be absorbed by the cold air, so immediately condenses into fog. At the same time the air is warmed by the sea causing the dew point to rise and the fog at sea level disperses.
The warmer air rises and is cooled again by the higher air so causing more fog to reappear. This fog effect resembles smoke, quickly forming, dispersing, and reforming. Lasting only a short time until the sea warms the air sufficiently eliminating the effect, sea smoke is not a major hazard to sailors.