Fog develops for a variety of reasons and a number of types can he identified:
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Advection fog develops as a result of a mass of warm air, with a high relative humidity value, moving horizontally (hence the term adveciion) over a cooler surface, whose temperature is below the dew-point temperature of the air. |
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As a result of conduction aided by turbulence, the air is cooled below its dew-point temperature. water vapor condenses, the water droplets producing the mist/fog condition. |
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This type of fog forms and persists under a wide range of wind speeds. The degree of turbulence dictates the maximum height to which the air is cooled, the height increasing with increasing wind speed. |
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At sea advection often termed sea fog, occurs at certain times of the year. In northern latitudes, the Grand Banks of Newfoundland and the North Pacific zones are notorious particularly in July, when warm air from the south-west and south pass over the cold waters of the Labrador, and the Oyo Shio or Aleutian Currents respectively. Sea fog also occurs in lower latitudes during the summer in the region of the cold California, Canary, Peru and Benguela Currents. |
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Sea fog not only develops where cold currents exist. but also where there are favorable conditions of wind speed, air and sea surface temperatures. Examples are the spring and early summer fogs of the Western Approaches to the British Isles, where the south-westerly warm air stream from the Azores moves over the sea which. at this time of the year, is at its lowest temperature. |
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In the North Sea. sea fog develops during the summer when warm north-east, east and sometimes south-easterly winds from Europe pass over the colder sea surface. Along the east coast of the British Isles this sea fog is called haar or sea fret. |
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On land, warm air moving over cold surfaces may also produce advection fog. In the British Isles this usually occurs in winter through advection of a warm air stream from the Azores. At this time of year advection fog also develops over the southern and eastern areas of the United States of America, when warm air is advected from the Gulf of Mexico and the Bermuda region. |
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Sea fog is a frequent threat to the seafarer and its prediction is therefore
important. As sea and dew-point temperatures are critical in its formation,
their observation at frequent intervals is recommended, and should he
recorded in graphical form. By drawing straight lines to establish the trend
of each temperature. it is possible to determine the point of intersection,
which indicates when fog may he encountered. |
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Sea smoke, arctic sea smoke, frost smoke, or steam fog is present when the surface of the sea has a steaming or smoky appearance. |
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This fog is often patchy and extends to a limited height above the surface, with good visibility at bridge level but poor from the upper deck. |
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The condition is caused by the movement of cold air over a warmer surface, the temperature difference usually being of the order of 10°C, although given favorable wind conditions it may occur with smaller differences. |
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The air immediately above the surface is heated and becomes saturated through evaporation from the surface. |
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It ascends and mixes with colder unsaturated air above. |
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Since the mixture is supersaturated, condensation occurs and the water droplets form sea smoke. |
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The wind speed associated with the formation of sea smoke may vary from very low to gale force. |
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Higher speeds are more favorable when the temperature difference is small, as they ensure a continuous supply of cold air immediately above the surface. |
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Off the cast coasts of the North American and Asian continents sea smoke occurs during the winter months, when cold air from the continent passes over estuaries, coastal waters, and adjacent ocean areas. |
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During winter it occurs in the Baltic Sea which is surrounded by a colder land mass, and in higher latitudes it is associated with cold winds from the Arctic Basin and the ice covered sea areas to the south. |
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In lower latitudes it occurs occasionally in the Gulf of Mexico and off Hong
Kong. |
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Radiation fog is a land based fog in its development. |
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Clear skies. a high relative humidity, very low wind speeds and a relatively long period during which the air can cool are the most suitable conditions for its formation. |
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The clear sky condition allows the maximum loss of long wave radiation from the surface during the night. |
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Surface temperatures decrease rapidly and the air immediately above is cooled through conduction aided by turbulence. |
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Once the air is cooled below its dew-point temperature, condensation occurs and radiation fog is produced. |
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Since the length of the cooling period is critical, radiation fog is more common during the autumn and winter in mid and high latitudes e.g. in the British isles. |
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Radiation fog will affect visibility at sea if it drifts over estuaries and coastal waters as a result of light offshore winds. |
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Radiation fog may disperse as a result of an increase in land surface temperature during the day, since the surface heats the air immediately above, and lowers its relative humidity. |
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An increase in wind speed can also cause dispersal since it overturns the air. |
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In tropical regions, radiation fog is comparatively rare at sea level, but may be experienced over river estuaries during the early hours of the morning. |
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The fog develops during the night over adjacent river banks, where the air has a high relative humidity due to the presence of open water. |
Source: Meteorology for mariners
Updated: 23 Sep 2007