FREQUENCY OF OCCURRENCE OF PARTICULAR CLOUD GENERA AT NOON AND DURING CERTAIN SYNOPTIC SITUATIONS IN CRACOW

PRACE GEOGRAFICZNE, zeszyt 105 Instytut Geografii UJ Kraków 2000 Dorota Matuszko FREQUENCY OF OCCURRENCE OF PARTICULAR CLOUD GENERA AT NOON AND DURING CERTAIN SYNOPTIC SITUATIONS IN CRACOW Abstract: The role of clouds, as a complex index of the condition and change of weather on both a local and global scale, is increasingly more visible. The aim of this study is to attempt to define the impact of circulation on the frequency of occurrence of particular cloud genera, separately for each month, through the calculation of the probability of occurrence of certain cloud genera, depending on synoptic situation. The study draws on the unique observation material from the period 1906 1995 (due to the uniformity of place, continuity and length of the series), which comes from the Cracow meteorological station of the Jagiellonian University. Key words: cloudiness, Cracow, atmospheric circulation, genera of clouds, synoptic situation. 1. Introduction The development of the global climate forecasting requires detailed information on clouds, as the initial data in the models assumes beforehand the occurrence of particular clouds (Maleshko, Wetherald 1981), and the factual nephological data may serve to verify the models forecasting of the cloud situation (Hansen et al. 1983). The observations of clouds at the Cracow station, which form an unbroken series for almost 100 years on the same site, may be applied to such a type of study. The aim of this study is to define the probability of occurrence of particular cloud genera and their frequency, depending on the synoptic situation. Apart from the cognitive factor of stating the interdependence of various air circulation types and cloud genera, it also has a forecasting aspect. It enables quantity forecasting of cloud genera on the basis of the circulation forecast. The analysis presented, is an attempt to construct a simple statistic model, which would forecast cloud genera in Cracow at noon, on the basis of the circulation type predicted. This study constitutes an appendix to the study by Niedźwiedź (1981) as well as Niedźwiedź and Ustrnul (1994), who only assessed the impact of the synoptic situation on the level of cloud

86 PRACE GEOGRAFICZNE, ZESZYT 105 Fig. 1. Percentage of cloud genera in Cracow (1906 1995). Ryc. 1. Udział procentowy rodzajów chmur w Krakowie (1906 1995). cover. The information on the cloud genera is especially important, the cloud genus is related to the thermal and hydrodynamic condition of the atmosphere, and impacts on the radiation or precipitation. The study employs the data on the cloud cover at noon, collected on a daily basis at the station of the Climatology Department, at the Institute of Geography of the Jagiellonian University in Cracow, in the period between 1906 1995, and for the analogous period the synoptic situations calendar by Niedźwiedź (1988 and amendments). Due to the large quantity of the material, it was impossible to use the data from all three climatological times. The noon time was chosen eliminating the two other times. The morning time is unfavourable to Cracow, as the town is located in an inversion valley, which encourages fog and low level stratus clouds, which cover the clouds at higher levels. The night time raises Fig. 2. Annual frequency of the Ac, As, St and Cu cloud occurrence in Cracow (1906 1995). Ryc. 2. Przebieg roczny częstości występowania chmur Ac, As, St, Cu w Krakowie (1906 1995).

FREQUENCY OF OCCURRENCE OF PARTICULAR CLOUD GENERA... 87 doubts, as the actual hours of night observation have changed throughout the years, and also due to possible misjudgements in defining the cloud cover after the sunset. The choice of noon time was also encouraged by the fact that in the foreign literature (McDonald 1938; London 1957; Seide 1954) one can find studies based exactly on mid day observations. The study includes the calculation of the cases of particular cloud genus occurrence and the cases of clear skies at particular synoptic situations, separately by month. Furthermore, the results were collected in tables, which allow one to define the probability of occurrence of particular cloud genera and their systems, for any given synoptic situation. The analysis describes the most common types of circulation in each month, and the probability of particular cloud genus occurrence for these types of circulation. 2. The frequency of occurrence of particular cloud genera and synoptic situations in Cracow The average data from the period between 1906 1995, shows that the biggest quota within the cloud cover of Cracow belonged to the following clouds (Fig. 1): Cu (20%), Sc (14%), Ac (14%), As (10%), Ns (9%), St (8%) and Ci (8%). Probably the occurrence of the low level clouds is influenced not only by circulation, but also by radiation and local conditions, as at other climatological times, in the same synoptic situation, the percentage of the particular cloud genera is different (Matuszko 1998). In the morning, St clouds dominate (18%), while at night Sc and Ac clouds prevail (15% each). The frequency of occurrence of the mid level and high level clouds does not change significantly, regardless of the time (1% difference). The fact suggests that these genera are the most dependent on circulation. The clear sky, in the yearly average, is most often observed in the evening (13%), and the least often at noon (4%), which is related to the convection development throughout the day (Matuszko 1991). The occurrence of particular cloud genera in Cracow is characterised by seasonal variability. The clouds marked by a clear annual Fig. 3. Annual wind rose frequency distribution of the air flowing over Cracow (1906 1995). Ryc. 3. Roczna róża częstości występowania kierunków napływu powietrza nad obszar Krakowa (1906 1995).

88 PRACE GEOGRAFICZNE, ZESZYT 105 Tab. 1. Frequency of occurrence (in %) of particular circulation types in the Upper Vistula Basin during the period 1906 1995. Tab. 1. Częstość występowania (w %) poszczególnych typów cyrkulacji w dorzeczu górnej Wisły w okresie 1906 1995. cycle, with their maximum in summer and minimum in winter, include Ci, Ac, Cu and Cb. The reverse cycle, with the maximum in winter, is observed for the following clouds: As and St (Fig. 2). The fundamental criterion for creating the synoptic situations calendar by Niedźwiedź (1988) was the direction of air masses advection and the type of the pressure pattern. The classification in which the author enumerated 20 types of synoptic situations allows for defining the circulation relations for each 24 hour period. The frequency of occurrence of the particular synoptic situations for the 90 year period between 1906 1995, has been presented in Table 1. In Cracow, on average,

FREQUENCY OF OCCURRENCE OF PARTICULAR CLOUD GENERA... 89 anticyclonic situations prevail (52.9%) over the cyclonic situations (45.1%). The dominance is visible every month, except April, when there are 10% more cyclonic situations than anticyclonic situations. The highest frequency (over 60%) of high pressure situations occurs in October and September, and the lowest frequency falls in April (44%). Among 20 types of synoptic situations, the most common is the Ka anticyclonic wedge (11.3%), slightly less predominant the Wc western cyclonic situation (10.6%), followed by the Wa western anti cyclonic situation and the Bc cyclonic trough. The stagnation weather is typical of the warm part of the year, and the high pressure reaches its peak values in August and July (16.2 and 15.9%), while the low pressure dominates in June and May (14.0 and 13.5%). However, in the cold part of the year advection situations from the west prevail, slightly more often (by 1%) with low pressure than high pressure, with the maximum values in December (16.3%). Other synoptic situations rarely occur, representing only a small percentage in a month. The Cc central cyclonic situation is the least frequent in a year (1.5%). The analysis of directions of the air advection (Fig. 3) indicates that, regardless of the pressure pattern, Cracow is dominated by the flow from the western sector (27%), north western sector (14%) and south western sector (12%). The eastern sector is also quite frequent (11%). The least frequent (7%) is the flow from the north east. Fig. 4. Cloud cover size at particular circulation types (acc. to Niedźwiedź 1994). Ryc. 4. Wielkość zachmurzenia w typach cyrkulacji (wg T. Niedźwiedzia, 1994).

90 PRACE GEOGRAFICZNE, ZESZYT 105 Tab. 2. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) at noon, with various synoptic situations in Cracow December. Tab. 2. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur i pogody bezchmurnej (Bch) w południe przy różnych sytuacjach synoptycznych w Krakowie grudzień. 3. The impact of synoptic situations on cloud cover in Cracow The studies conducted (Niedźwiedź, Ustrnul 1994) prove that the level of cloud cover depends strongly on the circulation. A significantly bigger cloud cover (exceeding 80%) occurs in Cracow at cyclonic situations (Fig. 4), especially with the NEc advection from the north east, the Nc from the north, the Ec from the east, and the SEc from the south east, as well as with the Cc non advection cyclonic situation. The least cloud cover (below 40%) in Cracow is related to the high pressure situations, with the

FREQUENCY OF OCCURRENCE OF PARTICULAR CLOUD GENERA... 91 Fig. 5. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) with Wa and Wc situations in Cracow December. Ryc. 5. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur i pogody bezchmurnej (Bch) przy sytuacjach Wa i Wc w Krakowie grudzień. Fig. 6. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) with Wa and Wc situations in Cracow January. Ryc. 6. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur i pogody bezchmurnej (Bch) przy sytuacjach Wa i Wc w Krakowie styczeń. SWa air flow from the south west, the Sa from the south and the Ca centre of the high pressure zone over the South Poland. In winter the least daily cloud cover (about 70%) is seen in Sa and SWa situations, while the biggest cover (exceeding 90%) occurs at the Ec and SEc low pressure situations (Niedźwiedź 1981). On the least cloudy days in December and January, the St and Ac clouds occur, while on the most cloudy days, the St and Ns clouds prevail (Table 2 & 3). In December

92 PRACE GEOGRAFICZNE, ZESZYT 105 Tab. 3. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) at noon, with various synoptic situations in Cracow January. Tab. 3. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur i pogody bezchmurnej (Bch) w południe przy różnych sytuacjach synoptycznych w Krakowie styczeń. (Tab. 1) the most frequent are the situations with advection from the west, mainly the Wc (16.3%) and Wa (12.5%). The Wc situations are most probably (Tab. 2, Fig. 5) accompanied by the Sc clouds (20%) and the Ac clouds (17%). The least probable (1%) is the occurrence of cloudless weather. The Wa high pressure situation is most often accompanied by the Ac and St clouds (16% each).

FREQUENCY OF OCCURRENCE OF PARTICULAR CLOUD GENERA... 93 Tab. 4. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) at noon, with various synoptic situations in Cracow February. Tab. 4. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur i pogody bezchmurnej (Bch) w południe przy różnych sytuacjach synoptycznych w Krakowie luty. January (Tab. 1) is also dominated by the advection situations from the west, where the high pressure situations (14.1%) are slightly less frequent than the low pressure situations (14.3%). At the Wc situation, the Sc clouds occur with a probability of 18% (Tab. 3, Fig. 6), and the second most probable clouds are the Ac clouds, with a probability of 17%. The cloudless weather in this situation may occur with the

94 PRACE GEOGRAFICZNE, ZESZYT 105 Fig. 7. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) with Wa and Wc situations in Cracow February. Ryc. 7. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur i pogody bezchmurnej (Bch) przy sytuacjach Wa i Wc w Krakowie luty. Fig. 8. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) with Ka and Wc situations in Cracow March. Ryc. 8. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur i pogody bezchmurnej (Bch) przy sytuacjach Ka i Wc w Krakowie marzec. probability of 1%. At the Wa situation, the least probable cloud is the Cb cloud (1%), while the most probable clouds are the Ac and Sc (14% each). In February (Tab. 1), the advection situations from the west also prevail, including the Wc (11.9%) and Wa (11.1%) situations. However, the high pressure situations are most often accompanied (Table 4, Fig. 7) by the Ac and As clouds (14% each), while the low pressure involves the presence of the Sc clouds (18%) and the Ns clouds (16%). In this month, cloudless weather is most often present at the Ca (28%) and

FREQUENCY OF OCCURRENCE OF PARTICULAR CLOUD GENERA... 95 Tab. 5. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) at noon, with various synoptic situations in Cracow March. Tab. 5. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur i pogody bezchmurnej (Bch) w południe przy różnych sytuacjach synoptycznych w Krakowie marzec. SEa (23%) situations. The St clouds dominating in January are encouraged by the Ec situation, with a probability of 38%. Summing up, winter is dominated by the air flow from the west, related to the activity of the Atlantic low pressure zone moving across Europe. This circulation is most frequently accompanied by the Ac and Sc clouds, connected with the polar and maritime air, as well as with the cold fronts which prevail in Poland (Warakomski 1962). The rather frequent (in winter) polar and continental air, brought by the high

96 PRACE GEOGRAFICZNE, ZESZYT 105 Tab. 6. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) at noon, with various synoptic situations in Cracow April. Tab. 6. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur i pogody bezchmurnej (Bch) w południe przy różnych sytuacjach synoptycznych w Krakowie kwiecień. pressure zone from the south east and east, encourages the cloudless weather or the low level stratus clouds. In spring, the least average daily cloud cover (40%) occurs at the stagnation high pressure weather and the advection from the south and south west (Niedźwiedź 1981). However, from March, the convection increases, causing bigger cloud cover during the day. The biggest cloud cover (exceeding 80%) occurs at the N and NEc situations.

FREQUENCY OF OCCURRENCE OF PARTICULAR CLOUD GENERA... 97 Fig. 9. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) with Ka and Bc situations in Cracow April. Ryc. 9. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur i pogody bezchmurnej (Bch) przy sytuacjach Ka i Bc w Krakowie kwiecień. Fig. 10. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) with Ka and Bc situations in Cracow May. Ryc. 10. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur i pogody bezchmurnej (Bch) przy sytuacjach Ka i Bc w Krakowie maj. In March (Tab. 1), the western cyclonic situation Wc still prevails (10.8%) along with the related Sc clouds (Tab. 5, Fig. 8), although the convection is more visible, as the Cu clouds occur with a similar frequency, and in the majority of high pressure situations they are the most frequent clouds to occur. April (Tab. 1) is dominated by the non advection Bc and Ka situations, and this is the only month where the low pressure situations occur more often than high pressure

98 PRACE GEOGRAFICZNE, ZESZYT 105 Tab. 7. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) at noon, with various synoptic situations in Cracow May. Tab. 7. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur i pogody bezchmurnej (Bch) w południe przy różnych sytuacjach synoptycznych w Krakowie maj. situations (by 10%). The increasing convection means that even at the Bc situation (Tab. 6, Fig. 9), the Cu clouds occur with the highest probability (17%), followed by the Sc (16%) and Ns clouds (14%). The high pressure situations, especially the Ka ones, are accompanied by the Cu (32%), Ac (13%) and high level clouds. In May (Tab. 1), once again the non advection situations prevail, including the Bc (13.5%) and the Ka (12.3%), although the anticyclonic situations occur rather frequently (9.1%), such as the eastern (Ea) and north western NEa (6.3%), causing

FREQUENCY OF OCCURRENCE OF PARTICULAR CLOUD GENERA... 99 Tab. 8. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) at noon, with various synoptic situations in Cracow June. Tab. 8. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur i pogody bezchmurnej (Bch) w południe przy różnych sytuacjach synoptycznych w Krakowie czerwiec. the re occurrence of low temperatures and ground frosts (Niedźwiedź 1988). In May, regardless of the circulation type, and due to the increasing convection, the most frequent clouds are the Cu (Tab. 7, Fig. 10) and the accompanying Ac and Ci. With the Bc cyclonic trough, the rain Ns (11%) and Cb (10%) clouds are often present, as well as the As clouds (11%).

100 PRACE GEOGRAFICZNE, ZESZYT 105 Tab. 9. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) at noon, with various synoptic situations in Cracow July. Tab. 9. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur i pogody bezchmurnej (Bch) w południe przy różnych sytuacjach synoptycznych w Krakowie lipiec. Therefore, in spring, local conditions plus the convection have the main impact on cloud formation, as the most frequently observed clouds are the Cu, which cover the clouds at a higher level. In summer the smallest cloud cover occurs in August (it is also the smallest average monthly cloud cover in a year 59%) at the Ea and SEa situations, and the biggest (exceeding 80%) at the Nc and NEc situations.

FREQUENCY OF OCCURRENCE OF PARTICULAR CLOUD GENERA... 101 Fig. 11. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) with Ka and Bc situations in Cracow June. Ryc. 11. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur i pogody bezchmurnej (Bch) przy sytuacjach Ka i Bc w Krakowie czerwiec. Fig. 12. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) with Ka and Wc situations in Cracow July. Ryc. 12. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur i pogody bezchmurnej (Bch) przy sytuacjach Ka i Wc w Krakowie lipiec. In June (Tab. 1), just as in spring, the stagnation situations prevail, including Bc (14.0%) and Ka (13.6%). The Ka situation (Tab. 8, Fig. 11) usually carries the following frequency of cloud occurrence: Cu (37%), Ac (13%), and Ci (12%). However, the Bc cyclonic trough, apart from the Cu clouds, includes the presence of the clouds accompanying the cold front, namely: the Sc (14%), Ac (13%) and Cb (11%). In July (Tab. 1), due to the increased activity of the high pressure zone over the Azores, the dominating phenomena includes the air flow from the west (Wc 11.5%,

102 PRACE GEOGRAFICZNE, ZESZYT 105 Tab. 10. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) at noon, with various synoptic situations in Cracow August. Tab. 10. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur i pogody bezchmurnej (Bch) w południe przy różnych sytuacjach synoptycznych w Krakowie sierpień. Wa 10.3%) and the high pressure wedge Ka 15.9%. These types of circulation are accompanied by the Cu and Ac clouds (Tab. 9, Fig. 12). July observes the maximum (5.6%) advection from the north, which encourages the air levelling up on the slopes of the Carpathians, and heavy rains from the Ns clouds (23%) and Cb clouds (15%), both in the Na and Nc genera, causing flooding in the Carpathian tributaries of the Vistula River (Niedźwiedź 1988).

FREQUENCY OF OCCURRENCE OF PARTICULAR CLOUD GENERA... 103 Tab. 11. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) at noon, with various synoptic situations in Cracow September. Tab. 11. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur i pogody bezchmurnej (Bch) w południe przy różnych sytuacjach synoptycznych w Krakowie wrzesień. In August (Tab. 1) the non advection types prevail (Ka 16.2%; Bc 11.5%), as well as the air flow from the west (Wa 10.4%, Wc 10.5%). The stagnation situations are accompanied by the Cu and Ac clouds, while the advections by the Cu, Ac and Sc clouds (Tab. 10, Fig. 13).

104 PRACE GEOGRAFICZNE, ZESZYT 105 Fig. 13. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) with Ka and Bc situations in Cracow August. Ryc. 13. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur i pogody bezchmurnej (Bch) przy sytuacjach Ka i Bc w Krakowie sierpień. Fig. 14. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) with Ka and Wc situations in Cracow September. Ryc. 14. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur i pogody bezchmurnej (Bch) przy sytuacjach Ka i Wc w Krakowie wrzesień. Generally, in summer the weather is shaped mainly by the high pressure over the Azores, and is characterised by cold fronts, the mp air masses and the air flow from the north. In autumn, the smallest cloud cover (ca. 40%) occurs in the Sa and SWa situations, due to the increasing foehn winds. The biggest cloud cover (exceeding 80%) is noted for the cyclonic situations, namely Nc, NEc, Ec, SEc, Cc and Bc (Niedźwiedź 1981).

FREQUENCY OF OCCURRENCE OF PARTICULAR CLOUD GENERA... 105 Tab. 12. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) at noon, with various synoptic situations in Cracow October. Tab. 12. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur i pogody bezchmurnej (Bch) w południe przy różnych sytuacjach synoptycznych w Krakowie październik. September (Tab. 1) is still dominated by the Ka high pressure wedge (13.9%), although the Wc western cyclonic situation occurs more frequently than Bc. The Ka anticyclonic situation (Tab. 11, Fig. 14) is also most often accompanied by the cloud cover consisting of the Cu (27%), Ac (17%) and Ci (10%) clouds, while the Wc low pressure weather is characterised by the following clouds: Cu (22%), Sc (17%) and Ac (16%). Of all the months, the cloudless weather predominates in September, which is encouraged by the anticyclonic situations, especially in the Ka and SWa situations.

106 PRACE GEOGRAFICZNE, ZESZYT 105 Tab. 13. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) at noon, with various synoptic situations in Cracow November. Tab. 13. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur ipogody bezchmurnej (Bch) w południe przy różnych sytuacjach synoptycznych w Krakowie listopad. October (Tab. 1) is a month where the high pressure situations largely prevail over the low pressure situations (by over 23%), encouraging the flow of the polar and continental air (the Ea and SEa situations). Such a circulation is frequently accompanied by cloudless weather or the St clouds. The most frequent situation in October, the Ka (11.6%), is favourable for the formation of the following clouds: Ac (14%), Ci (14%), Cu (14%) and Sc (13%) (Tab. 12, Fig. 15). The convection in this

FREQUENCY OF OCCURRENCE OF PARTICULAR CLOUD GENERA... 107 Fig. 15. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) with Ka and Wc situations in Cracow October. Ryc. 15. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur i pogody bezchmurnej (Bch) przy sytuacjach Ka i Wc w Krakowie październik. Fig. 16. Probability of occurrence (in %) of particular cloud genera and cloudless weather (Bch) with Wa and Wc situations in Cracow November. Ryc. 16. Prawdopodobieństwo występowania (w %) poszczególnych rodzajów chmur i pogody bezchmurnej (Bch) przy sytuacjach Wa i Wc w Krakowie listopad. month is less important, as the occurrence of the Cu clouds has a probability of 14%, while in September the probability is 27%. November (Tab. 1) is predominantly dominated by the western circulation (Wc 13.6%; Wa 11.0%), encouraging the formation of the Sc and Ac clouds. In certain types, the St clouds prevail (Tab. 13, Fig. 16), especially with the Wc, Sec and SEa situations. November is the month displaying the maximum frequency of the air flow from the south and south west, causing the foehn wind emergence at the Carpathian

108 PRACE GEOGRAFICZNE, ZESZYT 105 foreland. These situations (SWa) are most probably accompanied by the Ci clouds (20%) or cloudless weather (11%), the latter especially with the Ca and Ka situations. As the above results indicate, autumn is characterised by the dominance of the anticyclonic situations over the cyclonic ones, which results in smaller cloud cover, more frequent occurrence of cloudless weather and a higher probability of the occurrence of the high level clouds. 4. Conclusions This paper has two aspects: a prognostic one, and a cognitive one. The simple statistical model presented, analysed separately by month and displayed in tabular form, allows for defining in percentage terms the probability of the occurrence of particular cloud genera at noon, with a chosen (forecasted) synoptic situation. The cognitive aspect of this analysis lies in the statement of the impact of a circulation on the occurrence of particular cloud genera. Local conditions and radiation factors have a significant role, especially in relation to low level clouds. The Cu cloud is the most frequent from March to September at noon, due to the increased convection, regardless of the synoptic situation, as they sometimes cover the clouds of a higher level. This is undoubtedly encouraged by the pressure pattern, as especially in these months the stagnation weather prevails, i.e. the high pressure weather, which occurs most frequently in July and August. In summer, the Cu clouds are most often accompanied by the Ac and Ci clouds, connected with the anticyclonic situations and the weather shaped by the high pressure zone over the Azores. The cold part of the year is characterised by advection situations from the west, caused by the activity of the low pressure zone over the Atlantic, incurring large cloud cover. Such a circulation is most frequently accompanied by the Ac and Sc clouds, connected with the polar and maritime air, and the dominating cold fronts. In winter, the polar and continental air, brought on by the high pressure zones from the south east and east, encourage the occurrence of cloudless weather or the St clouds, especially in the morning and evening (Matuszko, 1998). In spring and summer, these clouds most frequently occur with the Bc, Wc and NWc situations. Throughout the year, the cloudless weather is most closely related to the Ka high pressure wedge, although it hardly ever occurs with the north east cyclonic situation. The Ns and Cb rain clouds accompany the Bc cyclonic trough from April to August, while in other months they are present with the Wc western situation, and the NWc north western cyclonic situation. The division into the standard seasons adopted, does not apply to the cloud cover aspect, as the weather (expressed here by the particular genera of clouds) is more strongly dependent on the particular synoptic situation than on an individual season. It means that the Ka high pressure wedge, regardless of the season, is most frequently accompanied by the Cu, Ac and Ci clouds, while the Bc cyclonic trough by the Ns and Sc clouds. Translated by Biuro tłumaczeń Letterman

FREQUENCY OF OCCURRENCE OF PARTICULAR CLOUD GENERA... 109 References Hansen J. et al., 1983, Efficient three dimensional global models for climate studies: Models I and II, Mon Wea. Rev., 111, 609 662. London J., 1957, A study of the atmospheric heat balance. Final Report, Contract AF19(122) 165 (AFCRC TR 57 287, New York University, [ASTIN117227]), 99. Mc Donald W. E., 1938, Atlas of Climatic Chartsof the Oceans, WB No. 1247, U.S. Gov t. Printing Office, 39 pp. and charts. Maleshko V.P., Wetherald R.T. 1981, The effect of a geographical cloud distribution on climate: Anumerical experimentwith an atmospheric general circulation model, J. Geophys. Res., 86, 11995 12014. Matuszko D., 1991, Występowanie nieba bezchmurnego i dni bezchmurnych w Krakowie, Zesz. Nauk. UJ, Prace Geogr., 83, 137 148. Matuszko D., 1998, Zachmurzenie w Krakowie. Przegl. Geof., XLIII, 3 4, 207 219. Niedźwiedź T., 1981, Sytuacje synoptyczne i ich wpływ na zróżnicowanie przestrzenne wybranych elementów klimatu w dorzeczu górnej Wisły, Rozpr. Habil. UJ, 58, 92 96. Niedźwiedź T., 1988, Kalendarz sytuacji synoptycznych dla dorzecza górnej Wisły (1951 1985), Zesz. Nauk. UJ, Prace Geogr., 71, 1 165. Niedźwiedź T., Ustrnul Z., 1994, Sprawozdanie z prac wykonanych w roku 1994 tematu M 2 pt. Ocena aktualnego stanu oraz zmienności klimatu Polski, maszynopis w IMGW. Seide R. N., 1954, The distribution of cloudiness by type and height in the Northern Hemisphere for spring and fall, M.S. thesis, New York University, 24. Warakomski W., 1962, O częstości występowania poszczególnych rodzajów chmur w Polsce, Przegl. Geof., VII (XV), 5, 185 192. Częstość występowania poszczególnych rodzajów chmur w godzinach południowych przy określonych sytuacjach synoptycznych w Krakowie Streszczenie Rozwój globalnego modelowania klimatu wymaga dokładnych informacji ozachmurzeniu, jako danych wejściowych w modelach zakładających a priori występowanie określonego zachmurzenia (Maleshko, Wetherald 1981), a rzeczywiste dane nefologiczne mogą służyć również do weryfikacji modeli prognozujących układ chmur (Hansen et al. 1983). Celem niniejszego opracowania jest próba określenia wpływu cyrkulacji na częstość występowania rodzajów chmur, poprzez obliczenie prawdopodobieństwa występowania poszczególnych rodzajów chmur w zależności od sytuacji synoptycznej, osobno dla każdego miesiąca w roku. W pracy wykorzystano unikatowy ze względu