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

Żmudzka E.

Przegląd Geofizyczny

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2005

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Z. 3-4

195-217

EN

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.

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Wielkość zachmurzenia w Polsce a epoki cyrkulacyjne

Żmudzka E.

Przegląd Geofizyczny

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2004

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Z. 1-2

25-42

EN

The aim of this paper is to determine long-term variation of certain characteristics of mean cloudiness in an annual course in Poland in years 1951-2000. Atmospheric circulation is seen as a determining factor of the cloudiness variation. The annual course of cloudiness is characterized by seasonal values, annual range and by winter and summer deviations from 50-year-means. The analysis performed shows distinctly that years of nephological changes in Poland are also years of boundaries between circulation epochs in different divisions of the second half of the 20th century (half of 60., half of 70. and around year 1983). This is because cloudiness is an element of climate determined mainly by circulation processes acting in a macro-scale. Most of the cloudiness variations in Poland can be explained by direct influence of atmospheric circulation; the west and the meridional components of atmospheric circulation on Central Europe and atmospheric pressure in the centre of this area can explain from 29% (turn of autumn and winter) up to 70% (July) of variability of mean monthly cloudiness values. In autumn and in the average situation in a course of a year the cloudiness depends mainly on the direction of air-mass flow - the meridional component. The role of the pressure values is similar to that of the west component. In other seasons the cloudiness is mainly determined by the atmospheric pressure in the centre of the Central Europe. Among directions of circulation in summer more important is a zonal component, however in spring and in winter - the meridional one. Complexity and seasonal diversification of circulation influence on cloudiness values (essential are: the atmospheric pressure pattern, direction of air-mass flow, frequency of fronts passes and their characters,...) and also often complex character of the circulation epochs make changes in characteristics of cloudiness in a course of a year in each epochs being not unequivocal (similarly as in a situation of air temperature and precipitation).

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Wielkość zachmurzenia w Polsce w drugiej połowie XX wieku

Żmudzka E.

Przegląd Geofizyczny

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2003

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Z. 3-4

159-185

EN

The paper presents the results of the research of the variability of cloudiness in Poland in the second half of 20th century. Average series of cloudiness in the area of Poland (excluding mountains) was analysed. The series was calculated based on data from 48 weather stations of the Institute of Meteorology and Water Management. The direction and rate of changes of cloudiness and its fluctuation were described. Particular attention was paid to anomalous values of cloudiness. Structure of atmospheric circulation, i.e. frequency of circulation types in season and years with anomalous cloudiness was determined.