7. Fog

7.1. General description

7.2. Heavy fog

7.1. General description

Fog, caused by the formation of condensed water droplets or ice crystals in the atmosphere is observed quite frequently in the Arctic [2, 13, 19, 30, 32, 42, 54]. It may be considered as a threatening or dangerous atmospheric phenomenon for various kinds of human activity primarily due to the reduced visibility (distances of 1 kilometers or less) which increases the probability of accidents for marine, ground and air transport [21, 40, 46, 47]. Fog can also amplify the effects of atmospheric corrosion.

All types of fogs are observed in the Arctic atmosphere: those consisting of liquid drops, ice crystals, and mixed particles (the latter two types are observed only when the air temperature is negative). The characterization of fog is based on the visibility as follows: weak (500 to 1000 meters), moderate (100 to 500 meters), heavy (less than 100 meters) and very heavy (less than 50 meters). Fog with 100 meters visibility or less is classified as highly dangerous (hazardous) if it extends over a sufficiently large area [51].

The most intense fogs form due to air temperature decreases, the main reason for the condensation of water vapor [41]. The following types of fog fall into this category: radiative, advective and orographic fogs (formed along sides of hills and mountains). Low cloudiness over the glaciers of the arctic islands and archipelagos as well as in the mountains is considered as fog, and frequency of occurrence is significant. Advective fog, which is characteristic of the marine Arctic, forms in relatively warm air masses that move over colder snow-ice or water surfaces as a result of non-adiabatic air cooling when air comes in contact with the surface. Radiative fogs in the Arctic are infrequent. They occur due to radiative cooling of ground surface and of the adjacent air layer. Near coastlines and atmospheric fronts, there are the fogs which form due to mixing of warm and cool air masses.

In some parts of the polar regions soaring fogs, which occur when cool air masses pass over warmer water or a highly humidity surface, are observed. Best known are the soaring fogs in the Kola Bay. In winter, local fog occurs quite often over polynyas in the sea ice. The amount of fog is related to the ice concentration [54].

A characteristic feature of the surface in the Arctic throughout most of the year is the presence of a snow cover that influences fog formation depending on the particular circumstances [41]. The most favorable conditions for fog formation over the snow cover are observed when the temperature is close to 0 degrees Celsius (from -5 to +5 degrees Celsius). This range of temperatures occurs throughout the summer in the Arctic basin. In this case, the difference between the saturation vapor pressure of H2O over water and that over ice is small. Under these circumstances, cooled air from the snow-ice surface is an important factor in fog formation. When air temperature varies from -8 degrees Celsius to -16 degrees Celsius, fogs occur less often and have lower intensity over a snow surface than over water or a surface not covered by snow, even when there are large temperature contrasts between the air and the surface. When the air temperature is very low, the fogs consist primarily of ice crystals and thus are in equilibrium with snow surface. As a result, the frequency of fogs increases again. According to observations, the layer thickness of fogs beneath the inversion can reach several hundred meters.

Particular properties, including the extent and frequency of fogs in the Russian Arctic, have been reported in a series of publications. Noteworthy among these are general conclusions produced by M. P. Kozlov [37] from the observational data of fogs at polar stations along the Northern Sea Route from 1932 to 1935. A general analysis of the results of observations up to 1952 were presented in the monograph of Z. M. Prik [54]. For subsequent years, the data have been reported in numerous climatic handbooks and studies of the Arctic prepared from 1960 to 1980. The current modern fog parameters for the Russian Arctic were derived at AARI in the department of meteorology based on observational data from 30 meteorological stations from 1936 through 1980, and extremes of fog parameters Their extreme characteristics have been charted using data up to 1985. In early 1990 the fog characteristics were added to the set of parameters calculated regularly for all polar stations. Information about fog statistics for the foreign Arctic has been published in [32 & 42] on the basis of which a map of the number of days with fogs in August and per year is presented in [42].

In the publications mentioned, heavy fogs were not specifically considered. Starting in 1988, data were collected and the frequency of heavy fog was analyzed at most of the Russian polar stations as part of the program for investigations of hazardous meteorological phenomena in the Russian Arctic. These results have been published in part in reference [38] in the section entitled "Hazardous hydrometeorological phenomena in the Arctic." In analyzing fog parameters, the average and extreme values were calculated from the 35-year meteorological data sets obtained at the polar stations. They were then used to interpolate over the entire Russian Arctic. For the foreign Arctic, average values appearing in various publications for 10-year observation intervals (or for shorter periods in some cases) were incorporated into the present analysis. The data from the continuous observations at the polar and drifting stations made it possible to obtain a more complete picture of the spatial distribution of fog events than in any of the works published earlier.

One of the characteristic parameters of fog is the spatial distribution of the number of days with fogs per month, season, or year throughout the Arctic. The spatial distribution of the number of days per year with fogs, shown in Figure 40, was obtained using the data from more than 150 stations including the drift stations. In different parts of the Arctic, the number of days with fog per year varies by about a factor of four, while the duration of the fogs differs by a factor of 10 to 12. Fog is observed most often in the arctic basin and over the arctic seas, where the average annual number of days with fog is 80 to 100. Fog is observed least often in the northwestern part of Canada and in certain mountainous regions (less than 10 to 20 days per year.

Figure 44

Figure 44. Contours of the number of days per year with fog.

Only about 20 percent of the total number of days per year with fog occur from October to April, which is less than 5 percent per month for that period. July accounts for 20 percent or more of days with fog. In some years, fogs are observed almost every day in summer, and the maximum number of days per year with fog reaches 150 to 160 (Table 43). Annual changes are less pronounced over the continent, and a summer maximum is not observed. In the Subarctic, the maximum number of days with fog is observed in winter while the minimum is observed in summer (Kyusyur Meteorological Station).

Table 43
Maximum number of days with fog in the Eurasian Arctic and Subarctic
Station Month Year
  Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Murmansk 11 9 11 4 4 3 7 8 8 7 10 16 59
Tersko-Orlovskiy 8 11 6 15 16 20 26 25 19 7 9 7 93
Rudolph Island 5 7 9 9 13 21 28 27 20 9 7 5 154
Cape Zhelaniya 7 10 19 8 19 23 28 26 18 6 4 4 132
Indiga 10 9 9 13 11 17 22 22 11 7 7 9 82
Amderma 10 8 10 13 15 22 26 28 22 10 14 8 128
Uyedineniya Is. 7 7 9 10 14 24 30 30 23 15 10 10 122
Belyi Island 13 12 12 16 19 20 26 24 19 19 12 10 128
Dixon Island 13 18 16 23 21 25 29 25 21 14 12 10 162
Cape Chelyuskin 10 6 13 13 10 22 27 28 20 7 10 12 105
Lake Taimyr 3 2 2 4 8 6 7 6 11 4 1 2 26
Khatanga 22 15 10 10 15 7 6 12 12 12 8 15 77
Preobrazheniya Island 11 8 9 10 13 20 24 24 21 6 7 9 105
Muostakh Island 2 2 2 5 15 20 22 17 12 3 5 3 70
Kotelíniy Island 5 6 9 12 14 24 25 26 20 8 8 7 99
Chokurdakh 8 6 5 4 11 13 11 17 14 13 9 6 62
Cape Schmidt 10 9 12 12 18 19 24 27 20 11 9 10 132

The average annual fog duration varies from 50 to 650 hours in the Eurasian Arctic. For the conditions existing in the ice-covered arctic seas, typical values are greater than 300 to 400 hours. From April through September fog duration ranges from 80 to 97 percent of the annual total. For the locations to the south which are far from the coast, the average fog duration is 50 to 200 hours, and the maximum does not exceed 300 hours. The duration of fog in the course of a day ranges from 5 to 7 hours for marine conditions and from 4 to 5 hours for continental conditions. Frequency of short-term fog events ("charges"), whose duration is about 1 hour, is 10 to 15 percent. The frequency of a fog duration that is less than 8 hours ranges from 80 to 90 percent. In some years, however, the fog duration period reached 4 to 5 days or more at the coastal and island stations. For example, as reported in [54], the fog duration was particularly long at Vaygach station (lasting from the 6th through the 15th of August 1932), at Yugorskiy Shar station (from 20 through 28 July 1915), and at Cape Chelyuskin Cape (from 4 through 14 August 1934). Other examples are also reported.

Under marine conditions, the minimum duration intervals of fog are typically observed in winter and the maximum in summer. The annual cycle of fog duration closely follows the variations in atmospheric water vapor and relative humidity (Fig. 45).

Figure 45

Figure 45. Intra-annual distribution of relative humidity (f) and fog duration (t ) in the Arctic: 1 - f for Vize Island; 2 - t (continental); 3 - t (marine).

The maximum fog duration period (48 hours or more) is observed over the marine part of the Arctic during the summer and autumn. In the western Arctic the frequency of such fogs per year varies from 0.02 percent (Golomyannyi Island) to 0.3 percent (Amderma). Daily fog duration is almost constant. In summer, fogs are often observed during the night and morning hours. Analysis of the statistical structure of the field for the number of days per year with fog using the empirical orthogonal functions method, showed that the first six eigenvectors span 70 percent of total variance [19]. The interannual variability in the number of days with fogs for marine conditions is associated with a coefficient of variation of 0.17 to 0.28, and for continental stations with a coefficient of variation increase to 0.4 to 0.5. Correlation coefficients for the number of days are less than 0.5 to 0.6 for neighboring stations. The standard deviation in the number of days per year with fog is 15 to 20 days for the seas and 5 to 10 days for the continental areas.

For practical applications, the temporal distribution of fogs for sparsely sampled areas of the Russian Arctic can be evaluated using the following empirical relationship derived from the polar station data.

t T = 6.7 nT ñ 57,

where t T is an average annual duration of fog at a station in hours; and nT is the average number of days per year with fog at that station.

To estimate the maximum number of days with fog, the following additional empirical formula can be used:

nmax = 1.33 nT + 14

where nmax is maximum number of days with fog per year.

7.2. Heavy fog

According to the "Protocol" [51], fogs are classified as highly dangerous when the visibility decreases to 100 meters or less. Cases of this kind were selected from the TM-1 tables from 1966 to 1985, and for certain stations up to 1989 to 1990. During the selection process all columns of "atmospheric phenomenon" tables were inspected. If fog was accompanied by a decrease in visibility to 100 meters or less, its date and time were recorded. After that, a composite chronology of dates with heavy fogs at particular polar stations was developed. Selection of data for the period after 1975 was carried out at Amderma, Dixon, Tiksi and Pevek hydrometeorological stations, and for 1975 or before, the data were compiled at AARI in the department of meteorology.

It is important to note that the criteria for the definition of heavy fogs, both visibility distance and duration, have changed several times in last 20 years. In addition, according to the present system, classification on the basis of instrumental observations with an index number of 00 code corresponds to a visibility of less than 100 meters, and an index number of 01 corresponds to a visibility of 100 meters [45].

For visual observations, additional gradations have been defined: code 90 means visibility less than 50 meters, code 91 means visibility 50 to 200 meters. The latter criterion includes visibility of 100 meters or less, which describes heavy and hazardous fog. At various polar stations, observations have been carried out using somewhat different methods. Within the 20-year observation period instrumental and visual observations were both performed at the same station. As a result, the data on heavy fogs used in this work are not completely uniform. But this non-uniformity did not cause a significant decrease in the quality of the generalized data, the number of days with heavy fog increased only slightly at some locations, and this was taken into account in the development of the corresponding charts and in the conclusions.

The main parameters used to characterize heavy fog are the number of days of heavy fog per month and per year. But a decrease in the visibility during fog events to 100 meters or less occurs infrequently. The results of the analysis of data from 65 stations on the distribution of the number of days with heavy fogs per year is presented in Table 44.

Table 44
Frequency of the number of days per year with heavy fogs in the Russian Arctic
Parameter Interval per category, days
0-1.0 1.1-2.0 2.1-3.0 3.1-4.0 4.1-6.0 6.1-8.0 >8
Number of stations 29 18 11 4 1 1 1
Frequency of occurrence, % 45 28 17 7 1 1 1

At almost half of the stations, the number of days with heavy fog for a year does not exceed 1.0, and more than 3 days with heavy fog are observed at only 10 percent of the stations. Additional detailed data on the number of days per year with heavy fog at the polar stations of the Russian Arctic distributed by month are presented in Table 45. Average number of days per year varies from 0.05 to 12 at different stations and times of year, a range of over 200 times. The spatial distribution of the average annual number of days with heavy fog over the Russian Arctic is presented in Fig. 46 a.

Table 45
Number of days with heavy fog at the polar stations
Station Month Year
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Rudolph Island         0.3 2.0 0.2 0.3 0.1       2.9
Cape Zhelaniya           0.05 0.4 0.2         0.7
Amderma       0.2 0.9 2.5 3.2 2.8 1.8 0.3 0.3 0.1 12.1
Belyi Island     0.05 0.2 0.2 0.9 0.9 1.0 0.4 0.2 0.05   3.9
Uyedineniya
Island
          0.1 0.3 0.2         0.6
Dixon Island       0.1 0.1 1.8 1.8 2.0 1.0 0.3 0.05   7.2
Golomyanny
Island
    0.1 0.05   0.1 0.6 0.3 0.2 0.1     1.4
Cape Chelyuskin           0.2 0.4 0.2         0.8
Lake Taimyr               0.05 0.05       0.1
Khatanga   2.1 0.2     0.1 0.1 0.2       0.6 3.6
Preobrazheniya
Island
            0.6 0.4 0.1 0.1     1.2
Tiksi         0.3 0.4 1.2 0.6 0.05       2.6
Muostakh Island         0.7 0.4 1.0 0.4 0.2       2.7
Kotelíny Island         0.1 0.2 0.3 0.4 0.05 0.2 0.05   1.3
Chetiryokh-stolbovoy Island         0.05 0.2 0.05 0.3 0.05       0.6
Ayon Island         0.1 0.2 0.2 0.1         0.6
Pevek           0.05 0.1           0.2
Wrangel Island           0.3 0.8 1.0 0.3       2.4
Cape Schmidt         0.2 0.4 0.2 0.8 0.3       1.9
Uelen             0.2 0.4 0.1     0.05 0.7
NP drifting stations   0.05       0.05 0.05       0.03 0.03 0.2

Note: In January the number of days with heavy fog is 0.3 in Khatanga and 0.03 for the North Pole stations.

Figure 46a

Figure 46b

Figure 46. Average (a) and maximum (b) number of days per year with heavy fog with visibility of 100 m or less.

The zones of maximum value (5 days or more) are observed in west arctic regions near the Karskie Vorota Straight and Yugorskiy Shar near the Dixon Island. In the eastern Arctic and the Subarctic the maximum values for the Kolyuchin Island and Cape Chelyuskin vicinity are somewhat less, approximately 3 days. The smallest number of heavy fogs was observed in the Siberia area, where a decrease in visibility to 100 meters at Kyusyur Station has been observed only once in 20 years.

In individual months and years, the number of days with heavy fogs can differ significantly from the mean values. The distribution of the maximum number of days per year with heavy fog is presented in Figure 46 b. The spatial distribution of the maximum values is, in general, quite similar to the distribution of the average annual number of days. The absolute values are different of course. In addition, there is only a small number of days with heavy fog in the central Arctic (Table 46).

Table 46
Chronology of heavy fogs in the central Arctic basin
Year Date Number of Days Station Station coordinates, degrees
N. Lat. E. Long.
1962 16 June
22 June
2 NP-11 79.0
79.1
194.3
195.4
1977 12 January
7 December
2 NP-22 83.1
80.3
234.0
231.0
1978 13 February
14 February
27 November
3 NP-22 75.3
75.3
73.2
226.4
226.4
201.1
1988 July 7
July 12
2 NP-30 76.4
76.5
161.1
160.5

The maximum values when the navigation routes are passable as well as for those within a year are of the greatest interest from a practical point of view. They are presented in Table 47. In areas where this phenomenon is most developed, the maximum number of days with heavy fog reaches 6 to 8 days in June and September, and 10 days or more in July and August. Significant maxima in the number of days with heavy fog were observed at Amderma Station, Dixon Island, in the Tiksi gulf and on Kolyuchin Island.

Table 47
Maximum number of days with heavy fog with 100 m or less visibility during the warm part of the year by month and for the year
Station Month Annual Total
Jun Jul Aug Sep
Rudolph Island 2 9 1 3 15 (1983)
Hayes Island 0 3 2 2 7 (1985)
Maliye Karmakuly 5 3 1 3 10 (1988)
Cape Zhelaniya 1 3 2 0 5 (1970)
Amderma 6 11 15 8 38 (1986)
Belyi Island 4 5 5 2 9 (1969,1973)
Yar-Sale 0 1 0 1 2 (1969)
Uyedineniya Island 2 2 1 0 2 (1975,1984)
Dixon Island 7 10 8 6 21 (1973)
Cape Sterligov 1 5 2 0 5 (1975)
Golomyanny Island 1 3 3 2 9 (1976)
Cape Chelyuskin 2 2 2 0 3 (1969,1979)
Lake Taimyr 0 0 1 1 1 (1970,1983)
Khatanga 1 1 1 1 10 (1976)
Malyi Taimyr Island 0 1 4 1 5 (1980)
Kyusyur 0 0 0 1 1 (1974)
Tiksi 2 10 4 1 18 (1966)
Kotelíny Island 2 2 3 1 9 (1969)
Yubileinaya 1 1 1 1 3 (1970,1971)
Kigilyakh 3 6 7 6 14 (1968)
Bunge Land 2 1 1 1 3 (1967)
Chetiryokhstolbovoy Island 2 1 2 1 3 (1968,1974)
Wrangel Island 1 4 4 2 9 (1977)
Cape Schmidt 2 1 3 2 4 (1977,1979)
Kolyuchin Island 3 7 10 2 24 (1966)
Uelen 0 2 4 1 4 (1977)
Cape Chaplin 5 5 6 1 12 (1968)
Provideniya Bay 2 6 2 2 6 (1985)
NP drifting stations 2 2 0 0 3 (1978)

Note: For the annual totals, the year of observation is given in brackets.

The intra-annual distribution of heavy fogs has its own individual characteristics at each station. As a whole, the distribution for the Russian Arctic, is characterized by a minimum in winter and a maximum in summer as a result of the seasonal changes in the meteorological processes and accompanying variations in ice conditions (Table 48). These results are determined on the basis of the data from 65 polar stations.

Table 48
The seasonal distribution of heavy fogs
Parameter Month
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Percentage of total No of days with heavy fog 1 1 1 1 6 18 28 28 11 3 1 1
Number of stations, where the heavy fogs were observed 7 7 14 17 35 51 61 59 52 24 10 10

Just 8 percent of heavy fogs occur in the 6 winter months (October through March), almost ¾ occur within the 3 summer months. In winter, heavy fogs were observed only at 10 to 30 percent of the stations, while in summer they were recorded at more than 80 percent of the stations.

Average annual number of days with fog in the Eurasian Arctic is closely connected with the fog duration (in hours). An analogous relationship apparently also holds for heavy fogs. To test this assumption, we used available corresponding data for the period 1975-1989 from 24 stations located on Franz Josef Land, the islands of the northeastern part of the Kara Sea, Severnaya Zemlya, the Taimyr Peninsula, and on some western islands of the Laptev Sea. It was found that the average annual duration of heavy fog events varied from 6 to 100 hours at different stations, and the number of days varied from 0.05 to 2.5. The following empirical relationship was obtained:

t 100 = 7.5 n100 ,

where t 100 is the mean duration of heavy fogs in hours, and n 100 is an average annual number of days with heavy fog.

Along the coasts and on the islands and archipelagos of the Russian Arctic heavy fogs with duration periods of 6 hours or more and visibility of 100 meters or less are not observed every year. From 1975 through 1989, heavy fogs were recorded only once at the Solnechnaya gulf and Lake Taimyr stations. They were recorded only twice at the stations on Rudolph Island, Krenkel observatory, Andrey, Isachenko and Pravda islands, Cape Sterlegov, Cape Peschanyi, Cape Chelyuskin, Provideniya gulf, and in Ust-Tarey. Three occurrences have been observed at the stations on Cape Leskin, Kosistyi, Uyedineniya and Krasnoflotskiye islands, Malyi Taimyr, and Preobrazheniya. Four such events occurred at the stations on Golomyanny and Geiberg islands; and five or more were recorded at the stations on Ushakov Island, Dixon, Izvestiya TsIK and Russkiy islands, and in Khatanga. The largest number of cases per year (11)was recorded on Dixon Island.

The longest total duration of heavy fog observed for a single year at selected stations is presented in Table 49. At most of the observation sites the maximum values did not exceed 20 to 30 hours per year. Only at 3 stations was the maximum duration more than 40 hours. The record of 99 hours in a single year was recorded on Ushakov Island which is located north of 80degrees N latitude. This value is slightly more than 1.1% of the entire year. The data presented in Table 49 show that the maximum annual duration of heavy fogs at various stations is spread out quite uniformly over different years during the investigation period. Only in 1983 was the maximum observed at a large number of stations (9), and at 3 stations in 1976 and 1986. The continuous duration of heavy fog is usually several hours. Data on 147 hazardous fogs over the sea coasts and Arctic islands show that the fog duration with visibility of 100 meters or less was 6 hours in 58 percent of the cases, 8 to 9 hours in 24 percent, and 10 to 15 hours in 14 percent. A duration of 15 hours or more was observed in only 4 percent of the total number of occurrences. Maximum duration of heavy fog was recorded at Ushakov Island (36 hours) on 7 to 8 July 1983, at Dixon Island (27 hours) on 8 to 9 June 1977, and at Izvestiya TsIK (18 hours) on 26 July 1979.

In the continental area of the Eurasian Arctic heavy fogs are rarely observed. Among the continental stations Khatanga is distinguished by its unique conditions. Here fogs with visibility between 50 and 200 meters are observed both in summer and winter due to the influence of anthropogenic influences (the airport and steam plants). The summer heavy fogs in Khatanga are of short duration, while in winter they are observed more often and for longer times. In 72 percent of cases they last longer than 10 hours. As a rule, the heavy fogs occurring in winter are observed when the air temperature is very low (below -40 degrees Celsius to -50 degrees Celsius), in conditions of atmospheric inversion and are classified as translucent ice fogs. Extreme cases with visibility decreasing to 50 meters or less have been observed. For example, on Hayes Island a fog of this density was observed for more than 3 hours on 5 August 1988.

The decrease in visibility due to hazardous fog levels over a substantial area poses the greatest threat for the national economic activity in the Russian Arctic. In the majority of cases, heavy fogs have been recorded on the same date at two or three stations, but very seldom at 4 or more.

From observations over the period 1975 through 1989 for the northeastern part of the Kara Sea and the Taimyr Peninsula, we find that heavy fogs occurred at two stations simultaneously in 74 percent of the cases, at 3 stations in 20 percent of the cases, and at 4 to 6 stations in 6 percent of cases. Non-hazardous fogs are observed simultaneously at multiple stations considerably more often. Twenty percent of the time in the Kara Sea, for example. Heavy fogs are observed at multiple stations most often in July and August - more than 72 percent of the time. The corresponding value for June and September in the Western and Eastern Arctic is about 20 percent. In winter the coincidence of heavy fogs at several stations is observed much less frequently.

Table 49
Maximum duration of heavy fogs with visibility of 100 m in the north-eastern part of the Kara Sea and on the Taimyr Peninsula. (1975-1989)
Station Maximum duration, hours Year
Rudolph Island 38 1983
Hayes Island 6 1976,1988
Ushakov Island 99 1983
Leskin Cape 18 1985
Uyedineniya Island 9 1983
Izvestiya TSIK islands 39 1978
Dixon Island 68 1977
Sterligov Cape 15 1975
Isachenko Island 15 1976
Ust-Tareya 15 1987
Golomyanny Island 18 1982,1983
Peschany Cape 18 1979
Pravda Island 12 1986
Russkiy Island 24 1983
Krasnoflotskiye islands 12 1983,1984
Geiberg Island 24 1985
Solnechnaya gulf 6 1976
Cape Chelyuskin 6 1983,1986
Lake Taimyr 6 1983
Khatanga 87 1986
Maliy Taimyr Island 27 1983
Andrey Island 9 1989
Pronchishcheva gulf 24 1979
Cape Kosisty 21 1989
Preobrazheniya Island 36 1980
NP drifting stations <48 1978

On the coast and islands of the Laptev, East Siberian and Chukchi Seas, the coincidence of heavy fogs with visibility of 100 meters or less at was observed to at 2 stations in 88 percent of all cases of simultaneous occurrence, at 3 stations in 8 percent, and at 4 or more in 4 percent. As a rule, the heavy fogs were recorded simultaneously at several stations for only a single day at a time. Only rarely and in the Western Arctic have such cases been observed for 2 to 3 days (26-28 September 1969; 16-17 July 1978; 8-10 August 1978).

Figure 47

Figure 47. Synoptic conditions associated with the occurrence of heavy fog on 21 July 1979.

Synoptic conditions during which the heavy fogs form are characterized, as a rule, by weakened atmospheric pressure fields. In this case, the anticyclone caused low speed winds which obstructed the rapid northward movement of a warm front (or its occlusion). By way of example, the situation occurring on 21 July 1979, when fog was observed in the northern Kara Sea and on Novaya Zemlya, is shown in Figure 43. A cyclone out of Western Siberia encountered a barrier - a high pressure area with its center over Franz Josef Land. Warm air reaching the cold surface gave rise to fog events of varying intensity. In other cases, heavy fog can form due to warm air flow from the south associated with anticyclones that move in from the west such that their centers are located over the arctic seas.

Table 50
interannual variability of the occurrence of heavy fogs over the north-eastern Kara Sea
Year Number of days per year with heavy fog at 2 or more stations simultaneously Year Number of days per year with heavy fog at 2 or more stations simultaneously
1966 0 1977 1
1967 0 1978 7
1968 1 1979 3
1969 6 1980 0
1970 3 1981 3
1971 1 1982 1
1972 0 1983 11
1973 2 1984 5
1974 0 1985 6
1975 4 1986 3
1976 5    

For a comparison of the available data on heavy fogs recorded at 2 or more stations, the event that occurred 18 August 1983 is considered here. Heavy fog was observed along the coasts and on the islands of the northeastern Kara Sea starting on 15 August, and proceeded to extend out across the Laptev Sea. By 18 August, the visibility had decreased to 100 meters or less at 8 stations distributed throughout the area from Severnaya Zemlya to the New Siberian Islands.

The number of days per year with heavy fog observed at several stations on a single day exhibits significant interannual variations (Table 50). There were 62 days in total with concurrent heavy fog events at two or more stations from 1966 to 1986, or 3 days per year on average. Deviations from the average number of days for the northeastern Kara Sea and the coast of Taimyr Peninsula range from -3 to +8 days (that is, 0 to 11 days per year). In this part of the Arctic the temporal variability of heavy fog can be characterized by background of short-term variability superimposed on a general trend toward an increase in occurrence over the time interval examined (1966 through 1986).