Sunshine Duration

Knowledge about sunshine duration, aside from being important theoretically, is also of practical significance. The study of sunshine duration enables improved calculations of global solar radiation (e.g. Spinnangr 1968; Dahlgren 1974; Markin 1975). Such a possibility is very important for the Arctic, where only infrequent numerous and short scries of actinometric observations are available. Sunshine duration, having a strong relationship with cloudiness, can also supplement our information, especially concerning the changes of cloudiness in the daily cycle. Registration of the duration of sunshine is a routine observation carried out in most meteorological stations. Bearing in mind all the above-mentioned facts, it is very strange that the dis tribution of sunshine duration in the Arctic is not better known. Bryazgin (1968) relates this to the surprisingly little attention which climatologists have paid to this element. During the last thirty years situation has not changed.

Some information about sunshine duration may be found in papers describing the climates of different (mainly small) parts of the Arctic (e.g. Meteorology..., 1944; Pettcrssen el al. 1956; Gavrilova 1963; Spinnangr 1968; Krenke and Markin 1973a, b; Markin 1975; Maxwell 1980; Pereyma 1983). Only Bryazgin (1968) has made a synthesis for the entire Arctic. In this paper he described the mean monthly and annual sums of sunshine duration, but only the maps for April and for the year as a whole are presented. April was chosen because, according to Bryazgin, the amounts of sunshine duration are greatest in this month. Later on, Bryazgin was a leading author of maps presenting sunshine duration in the Arctic, which are published in the Atlas Okeanov (Gorshkov 1980) and the Atlas Arktiki (1985). It is important to add that Bryazgin, in constructing these maps, used observational data from the period 1936-1970 (in Atlas Arktiki) from only 42 Russian meteorological stations. For the rest of the Arctic, the monthly and annual amounts of sunshine duration were computed based on a significant correlation between sunshine duration and cloudiness. So, for this part of the Arctic, the maps present only rough approximations of sunshine duration in reality.

Marshunova and Chernigovskii (1971), having data about sunshine duration for the whole Arctic, challenged Bryazgin's (1968) assertion that in the entire Arctic the maximum sunshine duration occurs in April. Bryazgin (1968) based his claim only on data from Russian stations and generalised the results from them for the whole of the Arctic. It turned out, however, that in the Canadian and Pacific regions the maximum duration of sunshine is observed in June or July. Even in Spitsbergen, as reported Spinnangr (1968), the highest values do not occur in April, but in May.

In January, sunshine duration does not occur above 70 N (polar night). In the lower latitudes of the Arctic, the monthly mean amounts of sunshine duration rarely exceed 10 hours.

In April, the amounts of sunshine duration, as mentioned above, are considerable (Figure 3.1). The maximum values of sunshine duration may be found in the vicinity of the North Pole and in the central part of Greenland Ice Sheet (> 400 h). A high duration of sunshine is generally typical of the whole Arctic Ocean, Greenland (excluding the southern coastal parts), and the northeastern parts of the Canadian Arctic (> 300 h), where anticyclonic activity prevails (low cloudiness) - see Figure 5 in Gavrilova (1959). On the other hand, the lowest amounts of sunshine duration (< 200 h) occur in the areas characterised by intensive cyclonic activity (the Atlantic region, southern part of (he Baffin Bay region, and (he southern part of Chukchi Sea and Bering Strait regions).

120 140 160'" 100 120J 140 160'

Figure 3.1. Sunshine duration (in hours) in the Arctic (after Alias Arktiki 1985).

120 140 160'" 100 120J 140 160'

Figure 3.1. Sunshine duration (in hours) in the Arctic (after Alias Arktiki 1985).

In July, the situation in comparison with April is dramatically worse, mainly in the Arctic Ocean, where, at this time, very high low-level cloudiness is observable (see Figure 5.4 or Vowinckel and Orvig 1970). Mean amounts of sunshine duration only vary about 100 h (four times less than in April). The same values, or even lower, are noted in the Norwegian and Barents seas. The Arctic shelves seas and coastal parts of the Russian Arctic receive about 150 h. Further to the south, the duration of sunshine rises to about 200-250 h. Bryazgin (1968) did not give the sunshine duration for the Canadian Arctic and Greenland. Dahlgren (1974) reported that the mean value of sunshine duration (from two years) at Devon Island in July was equal to 316 h. In Greenland, similar or (more probably) greater amounts occur.

In mid-October the polar night is present above 82"N. The Arctic islands above 80 N have only 1 h of sun. Further to the south the situation is better and coastal parts of the Arctic seas have about 20 h to 40 h and along the Arctic Circle about 60 h to 80 h (Bryazgin 1968). On Devon Island the sunshine duration is equal to 25 h (Dahlgren 1974) and is almost twice as great as in Spitsbergen (13 h, Isfjord Radio, 1951-1960 (Spinnangr 1968)). According to data published by Gavrilova (1963), it seems that the coastal parts of the Kara and Laptev seas have lower sunshine duration amounts in Octobcr (< 10 h) than were given by Bryazgin.

On an annua! basis, the inner parts of Greenland receive the greatest amounts of sunshine duration (> 2400 h) (Figure 3.1). Aside from Greenland, the south-western part of the Canadian Arctic and the western part of the Chukotka region receive more than 2000 h of sun. Almost the whole Canadian Arctic also has high values (> 1600 h), along with some southern parts of the Russian Arctic and Alaska. The lowest duration of sunshine occurs between Jan Mayen Island and Novaya Zemlya (< 800 h). Bryazgin (1968) distinguished a small area between Jan Mayen and Bjornoya islands where annual amounts of sunshine duration do not excced 300 h, Of all the Arctic stations, the lowest values are found in Bjornoya (annual mean only 249 h), then at Hopcn and Jan Mayen (both 444 h). This is the region with the highest cyclonic activity and, as a consequence, also the highest cloudiness in the Arctic.

The average mean relative sunshine duration for the whole Arctic, which indicates the percentage of the astronomically possible sunshine, oscillates between 20% and 25%. Throughout the year, the highest fraction of actual to possible duration of sunshine is recorded in March and April (30%-40%) and the lowest in October and November (3%-5% in the marginal Arctic seas and 15%—20% in the continental Arctic) (Gavrilova 1963).

There is a considerable fluctuation in the duration of sunshine in some years. From the comparison of the maximum and minimum annual values of this element given in Atlas Arktiki (1985), it is evident that the maximum values may be two to three or more times greater than the minimum values.

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