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Związek występowania dni pogodnych i pochmurnych w Polsce z polem ciśnienia nad Europą i Północnym Atlantykiem

Żmudzka E.

Przegląd Geofizyczny

2005

Z. 3-4

195-217

The study determines frequency of clear (N<25%) and overcast (N>75%) days over Poland and favourable barometric situation over Europe and Northern Atlanitic. Additionally the mean pressure partern on cloudless and on total cloudiness days was analysed. In order to determine the amount of cloudiness over Poland (except the mountains) average daily records of sky coverage from 16 synoptic stations were used as bases for calculating the spatial average cloudiness (N) for each day of the 35-year period (1966-2000). For the sake of description of air pressure, its average daily values were used. The values were taken from the nod-points of the grid form the area q 30°-70°N and A 40°W-60°E. The grid step is 5° latitude and 10° longitude. The number of overcast days is 7 times higher than the number of clear days. In the period 1966-2000 clear days accounted for 5.9% of all days which gives the number of 22 days on average in a year (from 1 day in November to 3 days in August and May). Overcast days accounted for 45.4% of all days - 166 days on average in a year (from 8 days in August to 21 days in December). Clear days generally occurred as single days, overcast days as few days sequences. The cloudamount is essentially connected to the atmospheric pressure over Central Europe. In winter, during increased cyclonic circulation, which is reflected in occurrence of trough of low pressure or low pressure centres in baric field over Central Europe, there are favourable conditions for development of big cloudiness over Poland. Similar pressure pattern favours overcast days also in summer, however this occurs rarely and therefore the cloud amount in summer is less. Build-up of high pressure centre over Poland or over neighbouring area with wedge of high pressure over Poland restricts development of cloudiness. Such situation is more frequent in summer, but definitely higher pressure occurs during "clear" and especially "cloudless" highs in winter. Spatial pattern of the highest and the lowest pressure centres in particular months and seasons, on clear as well as on overcast days, is subject only to slight changes or is stable in comparison with mean pattern. The mentioned spatial pattern is different only in case of barometric maximum on clear days during the cold part of the year, when the number of such days is least. The highest pressure is then connected to the high pressure over Poland and not to the high pressure area over Eastern Europe, as it is in the long-term case. During overcast days maximal pressure is sometimes even higher than during clear days, when the high pressure builds-up over Poland. Such situation happens in summer. Therefore it can be stated that the occurrence of clear or overcast days is mainly determined by the large-scale spatial pattern of pressure, which influences the direction of advection over particular area, thus a type of air-masses.