Generally speaking, because of the low air temperatures, water vapour contcnt is also low throughout the Arctic. This results both from limited evaporation and the small amount of water vapour which can be held by the cold air. The annual course of the water vapour pressure is therefore very similar to that of air temperature. In winter months, from November to March, and in some parts of the Arctic even to April (e.g. Spitsbergen), the water vapour pressure is the lowest and shows clear uniformity (Figure 6.1), although the day-to-day changes are almost the greatest in the annual course (see Przybylak 1992a). The values of this element in the Arctic must be the lowest over the Greenland Ice Sheet and probably oscillate near 0.0-0,1 hPa. They are also very low (about 0.2 to 0.6 hPa) in the coldest continental parts of the Canadian and Russian Arctic. In the parts of the Arctic characterised by an oceanic climate (Atlantic, Baffin Bay and the southern Pacific regions) the water vapour pressure oscillates between 2-4 hPa (Zavyalova 1971; Przybylak 1992a).
Figure 6.1. Mean annual course of relative humidity (0. water vapour pressure (e), and .saturation deficit (Ae) in Homsund (Spitsbergen) of days of various degree of cloudiness (C) in the period Nov. 1978 - Dec. 1983 (after Przybylak 1992a).
In Spitsbergen, the highest mean daily values rarely exceed 6 hPa and the lowest do not drop below 0.3 hPa. From April (or May) to June a significant increase in water vapour pressure is observed. The annual maximum of this clement occurs in July or August. During these months in the southernmost parts of the continental Arctic, the mean water vapour pressure can exceed 9 hPa (e.g., in Coppermine 9.5 hPa and 9.4 hPa in July and August, respectively). In the southernmost maritime areas the water vapour pressure is a little lower becausc the air temperature is also lower. The highest values occur here mainly in August and reach about 8 hPa (Bjornoya - 7.9 hPa, Jan Mayen - 8.0 hPa). In south-western Spitsbergen (Hornsund), the water vapour pressure is lower and the maximum is observed in July (7.1 hPa) (Przybylak 1992a), The highest mean daily water vapour pressure can reach 12-13 hPa in the southernmost parts of the continental Arctic and 10-11 hPa in the maritime Arctic. In Hornsund the maximum mean daily value for the period 1979-1983 amounts to 9.9 hPa and was connected with long-term (two weeks) inflow of warm and humid air masses from the southern sector. In September and October the greatest declinc of water vapour pressure is observed. From Figure 6.1 it can be seen that in Hornsund in all months (except August) the highest water vapour pressure occurs on cloudy days. This results from the fact that cloudy days (except during the high summer months) arc warmer than partly cloudy, and particularly, clear days. To explain the relationships between cloudiness and water vapour pressure, one can add that cloudy days are connected with the maritime (warm and humid) air masses coming from the southern sector, while clear days occur when Arctic or polar continental (cold and dry) air masses flow in from northern and eastern sectors. The mean vertically integrated meridional vapour flux over the Arctic for the surface - 700 hPa layer is clearly evident on the maps presented here (Figure 6.2). Poleward moisture transport in all months is the greatest in the Atlantic region where it is associated with cyclonic activity along the primary North Atlantic storm track. On the other hand, the equatorward fluxes dominate over the Canadian Arctic, particularly in July. The result of this is the opposite influence of atmospheric circulation on air humidity in the Atlantic and Canadian regions. In the former it leads to an increase of the absolute content of water vapour, while in the latter it results in a decrease.
The mean monthly daily courses of water vapour pressure in the cold half-year in Spitsbergen arc almost uniform, particularly during the polar night, becausc they depend on non-periodical factors. The highest and lowest values can occur with the same probability in every hour of the day. In the summer months, the differences in the fluxes of solar radiation reaching the Arctic in a daily cycle determine the occurrence of the maximum values in the afternoon and the minimum values of water vapour pressure in the "night" hours. However, their range is very small and does not exceed 0.3 hPa (Przybylak 1992a), The daily cycles are better developed on clear days than on cloudy days.
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