Fog

Four types of fog can be distinguished in the Arctic:

— "steam fog" or "Arctic smoke",

The most common type is advection fog occurring mainly in summer (particularly from June to September) when relatively warm, moist air flows in over a cold surface. The most favourable conditions for the formation of this type of fog arc the open waters of the Kara, Laptev, East Siberian, and Chukchi seas. Warm water carricd by the northern extension of the Gulf Stream system significantly reduces the frequency of this type of fog over the Norwegian and Barents seas. The frequency of these fogs decreases rapidly from the coastline inland and diminishes less rapidly over the pack ice. Fogs of this kind and other types also do not occur at wind speeds above 10 m/s (Vowinckcl and Orvig 1970).

The sccond type of fog - radiation fog - occurs mainly in winter, when small cloudiness favours large long-wave upward radiation. This radiation cooling is more effective in producing fog over coastal and inland areas because here the flux of subsurface heat is significantly smaller than over the oceanic ice. Radiation fogs are usually shallow and have a light density because they occur in very low temperatures.

The third type of fog - "steam fog" - is not very often observed in the Arctic. This fog can be found mainly over open water during the advection of very cold air. The necessary condition for its occurrence is the great contrast between air temperature and water temperature. Under such conditions the flux of water vapour to the atmosphere is greater than the surface cold air is able to hold. As a result, the excess moisture quickly condenses into fog. This fog occurs most frequently over rivers, unfrozen lakes, open leads, or polynyas. This type of fog docs not "live" long; it is very quickly dissipated by wind. Therefore, it is seldom of any great horizontal or vertical extent.

The fourth type of fog - ice fog - is formed when air temperature is low enough (usually below -30°C) to cause direct atmospheric sublimation of moisture in the form of ice crystals. Light ice particles with small full velocity remain suspended in the stagnant air near the surface for a long time. This type of fog occurs mostly near inhabited areas, which are a local source both of moisture and pollutants. Stable air-conditions (little or no wind) during cold spells lead to a large concentration of atmospheric pollutants. Therefore, the ice fog is considered to be one of the types of air pollution (Benson 1969; Maxwell 1982). Its thickness usually oscillates from about 15 m to 150 m. For more details see Berry and Lawford (1977) and Maxwell (1982).

During the course of the year, fogs occur with the highest frequency in the summer months. For example, in the Arctic Ocean their frequency is very high and ranges mainly from 65% to 80%. On the other hand, in winter fogs are observed very rarely (5-10%). However, in some areas of the Arctic a maximum frequency can also occur in every month. Inland areas, where radiation type fogs dominate, show a maximum during one of the cooler months and particularly during autumn (Petterssen et al. 1956). For example, in the Eismitte station (centra! Greenland) maximums occurred in October (26 days) and December (20 days) (Georgi et al. 1935). Moreover, Prik (1960) revealed that the occurrence of fog depends on the sea-ice concentration (Figure 5.6). The highest frequency of fogs occurs over the sea areas which have a 70-90% sea-ice cover. If there is less or more ice, a decrease in fog frequency is observed. The duration of fogs is different. Usually they do not occur for very long (< 6-12 h), but sometimes their duration can reach as much as 76 h (Prik 1960).

Days

0-1 2-3 4-5 6-7 8-9 10 Degree of ice concentration

Figure 5.6. Average number of days with fog during 10-day periods in summer months (July and August) for different degrees of ice concentration (after Prik 1960). I - Ostrov Ruskiy, 2 - Ostrov Uedinenya.

Information about geographical distribution and also about other characteristics of fogs in the Arctic is somewhat limited. Most existing publications analyse this atmospheric phenomenon for individual points or small areas (e.g. Loewe 1935; Bedel 1956; Kanevskiy and Davidovitch 1968; Krenkc and Markin 1973a, b; Pietron 1987). Among the works giving more a comprehensive and general insight into the problem one can mention Rae 1951, Pctterssen et al. 1956, Prik 1960, Andersson 1969, Ukhanova 1971, and Maxwell 1982. Surprisingly, such well-known studies as those of Vowinckel and Orvig (1970) and Putnins (1970), give very short notes not exceeding half a page. Only Prik (1960) presents maps with the frequency of fogs in the whole Arctic (for July) and Ukhanova (1971) for the non-Soviet Arctic (for July and for the year as a whole). The isolines must be treated, however, as a rough approximation because, for example:

1) the network of stations was too sparse,

2) different definitions of fogs were used,

3) observations were made with different frequencies in the course of a single day.

The accuracy of fog observations is greatest in the warm-half year. In winter, particularly during the polar night, the reliability of observations is lower. In this season, and also during the night hours in the transitional seasons, a spurious increase of fog frequency was noted (Sverdrup 1933).

In January, in the non-Soviet Arctic, the highest mean frequency of fogs occurs in Alaska, where it can exceed 20%, as in the case of the Barrow station (21%). In the rest of the area studied (excluding the inland region of Greenland) the frequency is lower than 10% (i.e. fewer than three days with fog). In the majority of the Canadian Arctic Archipelago and Greenland stations there is less than one day of fog in January.

In July (Figure 5.7), as was mentioned earlier, the frequency is at its highest almost everywhere. In the central part of the Arctic about 20 25 days with fog have been noted. A similar frequency also occurs in the northern part of the Barents and Chukchi seas, and probably in the East Siberian and Laptev seas (see Prik 1960). Over land areas, (he frequency significantly decreases by up to 5-10 days. A smaller number of fogs are also observed in the southern part of the Atlantic region (10-20 days).

The annual number of days with fogs is greatest in the central part of the Arctic (> 140 days). Over almost the entire Arctic Ocean there are more than 100 days with fog. Frequent fogs resulting from the cold East-Greenland Current are also noted between Greenland and Spitsbergen. The northern parts of all the Arctic seas also probably have more than 100 days with fog (except the Norwegian Sea and Baffin Bay). In the central part of Greenland, during (he Wegener expedition in the years 1931/1932, as many as 133 days with fog were recorded. However, two thirds of these fogs were due to the presence of clouds at the ice surface, and the remaining third were radiation fogs (Putnins 1970). Moreover, information from just one year is also rather unreliable. For example, on the Frcnch station Centrale situated not very far from the Eismittc station in 1949/1950, only 56 days with fog were observed. On coastal stations, the frequency of fogs is significantly lower and most often oscillates between 30 and 60 days, only rarely exceeding 100 days in some stations like Barrow (Ukhanova 1971), Ostrov Hejsa (Krenke and Markin 1973a, b). Krenke and Markin (1973a, b) showed that occurrence of fogs depends significantly on local conditions. More fogs are usually observed in more open coastal stations.

Figure 5.7. Average number of days with fog for the year (a) and July (b) (after Ukhanova 1971). 1 - average number of days with fog, 2 - maximum number of days with fog.

Chapter 6

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