Both first winter months of the year 2002 were markedly warmer, January by +1.3 �C and February even by +4.7 �C above the long-term normal. Moreover, in February the record maximum temperatures were exceeded several times in Prague – Klementinum where regular measurements have been carried out continuously since 1775. February of the year 1995 had similar temperatures and the warmest February over the recent 25 years was in 1990 with the deviation from normal +4.9 �C. Low precipitation, only 66 % of the long-term normal, occurred in January. Though the January precipitation was only 44 mm on the average, the snow cover in the mountains, thanks to abundant precipitation in November and December 2001 reached from 100 to 240 cm, and at some sites even more.

Temperature and precipitation conditions in the winter months by the end of the year 2002 had marked fluctuations. While temperatures in November were higher than the long-term normal (deviation +1.9 �C), the December deviation was -2.0 �C. The monthly total precipitation in November 2002 reached 147 % of the long-term normal. On the contrary, the December precipitation was only slightly above normal. The snow cover formed in late December 2002; the lowlands had up to 8 cm, the mountains about 10 cm and the Krkonoše Mts. 30–35 cm.

The character of the November weather was prevailingly cyclonal, similarly as in October. In December, on the contrary, the number of anticyclonic situations increased and the relation between the occurrence frequencies of the both types was approximately equal. The above-normal temperatures in November 2002 were caused by the fact that from 11 November to 4 December 2002 there was the leading pressure formation – cyclone with the centre above England. On its front side warm air from the south came to central Europe in the given period. Maximum daily temperatures amounted largely above 10 �C and the highest temperature 20.7 �C was measured on 16 November 2002 in Lučina (Frýdek-Místek).

In the beginning of the year 2002 warning signals were issued within the Smog Regulation System (SRS) in the Ostrava region and in Prague. In other monitored areas (northern Bohemia, the foothills of the Krušné hory Mts. and the Mělník region) there were no SRS signals issued. In the Ostrava region the warning signal was issued on 4 January 2002 at 11.20 a. m. On this day formation of a mightier inverse layer above the whole territory of the Czech Republic was observed. For instance, the ground-level midnight temperature in Prague-Libuš on 4 January 2002 was -14.3 �C, while in the level of 834 hPa the measured value was markedly higher – -7.9 �C. The meteorological situation was typical for the formation of unfavourable dispersion conditions. Increased concentrations of nitrogen oxides but also of sulphur dioxide and respirable particles PM₁₀ were measured. This situation lasted all day long on 5 January 2002, only after the passage of the southern edge of the warm front over northern Moravia and Silesia the mighty inverse layer was dissolved, the air mass was exchanged and, consequently, the pollutants� concentrations decreased. Therefore the warning signal could be cancelled on 6 January at 5.30 a. m. At the beginning of the period, covered by the issued warning signal, the highest nitrogen oxides concentrations slightly exceeded 300 μg.m^-3 and gradually began to decrease at the end of the first day to the usual level.

In Prague the warning signal was issued in mid-January 2002, when the large area of high pressure remained above the central Europe; along its rear side at higher levels warm air entered the country�s territory. This resulted in the formation of temperature inversion, and, consequently, in increasing the values of pollutants� concentrations in the ambient air, mainly of nitrogen oxides. The alert threshold limit value was exceeded at six stations at 12.00 a. m. on 14 January 2002 and with regard to weather conditions the warning signal was issued at 5 p.m. pursuant to the regulation rules for the city of Prague. The frontal system moved on from the west on 15 January 2002 and after its passage over the territory of the Czech Republic on the following day (16 January) the inversion was dissolved and the values of nitrogen oxides concentrations decreased. The warning signal was cancelled by the municipal authorities of the city of Prague on 16 January 2002 at 9.00 a. m. Maximum half-hour NO_x concentration was measured at Braník station – 431 μg.m^-3, while the highest three-hour concentrations did not exceed 294 μg.m^-3. With regard to the expected improvement of the dispersion conditions the regulation signal was not considered. At the end of the year 2002 no signals within the Smog Regulation System had to be issued in any smog area of the Czech Republic.

The summer of the year 2002 was characterized by significant number of cyclonal situations which occurred in 64 % of the days between 1 June and 15 September. As regards temperatures, this period can be generally characterized as slightly to moderately above normal, as regards precipitation more or less normal, with the exception of the month of August, which was highly above normal in Bohemia, and slightly above normal in Moravia and Silesia as concerns precipitation. Heavy above-normal rainfall resulted in disastrous floods in the Vltava and Labe river catchments. The amount of sunshine was around normal, with the exception of the month of August which was slightly subnormal on the major part of the republic�s territory.
From early May to late August the occurrence of increased and high concentrations of ground-level ozone are generally highly probable. From the meteorological viewpoint, these ozone episodes are connected with anticyclonic situations with prevailing warm and dry weather. The analysis of the weather in the summer period of 2002 shows that the conditions for the creation of summer photochemical smog episodes were not favourable. Anticyclonic situations in the monitored period had short-term character (lasting mainly from 1 to 4 days), with the exception of the last decade of July 2002, when at first western, later eastern anticyclonic situation occurred for 10 days. Western anticyclonic situation lasted from 20 to 26 July and was characterized by occasional invasions of colder air from northwest. Therefore, as regards temperatures, the mentioned period is characterized as subnormal.

In 2002, the ground-level ozone concentrations on the territory of the Czech Republic exceeded the information alert threshold 180 μg.m^-3 on 5 days in total. At solely one station during one day, this limit value was exceeded on 19 June in Plzeň (186 μg.m^-3) and on 20 June in Karviná (186 μg.m^-3). On 10 July 5 stations recorded ozone concentrations above the limit value: Tušimice 181 μg.m^-3, Most 207 μg.m^-3, Teplice 181 μg.m^-3, Rudolice v Horách 197 μg.m^-3 and Martiněves 187 μg.m^-3. On 13 July the limit value was exceeded again at one station only, and namely in Rudolice v Horách (maximum measured value was 199 μg.m^-3). The last ozone episode of the year 2002 occurred on 23 August with the following concentrations measured at the stations: Karviná 202 μg.m^-3, Studénka 188 μg.m^-3, Bílý Kříž 182 μg.m^-3, Rýchory 196 μg.m^-3 and Šerlich 180 μg.m^-3. The ozone alert threshold value was almost reached also at the beginning and at the end of the summer period: on 17 May 2002 in Hradec Králové and on 22 August 2002 in Karviná and Bílý Kříž concentrations about 175 μg.m^-3 were measured. The course of meteorological conditions and ozone episodes occurrences in summer 2002 are shown in Fig. 2.4.1.

Synoptic situation during ozone exceedance episodes on 10 July was determined by the cyclone of 1000 hPa with the centre above Scotland and by the anticyclone of 1020 hPa with the centre above the St. Petersburg region. Warm air from the south entered central Europe between these pressure formations. The cold front above western Europe undulated and slowly moved to the east and in late afternoon hours and during the night it passed over Bohemia, followed by cold air mass. The sky over the whole territory of the Czech Republic was clear, in the afternoon cloudy from the west in Bohemia, in the evening and at night there were storms and rain showers. Maximum afternoon temperatures in western Bohemia amounted to 28 �C, the rest of Bohemia had 32–34 �C, Moravia and Silesia 35 �C. Daily precipitation totals in Bohemia ranged from 2 to10 mm, locally 20–40 mm.

The highest daily NO₂ concentration 115 μg.m^-3 was recorded at Prague-Smíchov station and the lowest value 4 μg.m^-3 was measured at Bílý Kříž. Concentrations from 40 to 80 μg.m^-3 were characteristic for larger cities, in small towns they ranged about 20 μg.m^-3. The stations in higher altitudes measured 5–15 μg.m^-3 of nitrogen dioxide. The highest ozone concentration 207 μg.m^-3 was in Most, the lowest 105 μg.m^-3 at the stations in České Budějovice and Olomouc. Spatial distribution of maximum hourly ozone concentrations shows the marked area of the increased ozone concentrations from Prague to the northwest with maximum value of 207 μg.m^-3 in Most. Due to the direction of air flow it can be expected that ozone contributions to the regional level registered at the stations Martiněves, Most, Tušimice, Teplice and Rudolice result from emission of precursors from the Prague agglomeration, Kralupy and Záluží. In the direction from Znojmo to Prague a belt of concentrations above 150 μg.m^-3 was observed with the maximum level of 170 μg.m^-3 in Košetice. In this case it can be regarded as the northern edge of �an urban type plume� coming from Vienna. In previous years airborne measurements of ozone and its precursors confirmed the existence of such plumes in Vienna and Prague leeward sides. (Figs. 2.4.2 and 2.4.3)
On 23 August warm air from southeast came to central Europe in the rear side of the anticyclone of 1020 hPa with the centre above southern Scandinavia. The cold front above western Europe undulated and slowly moved to the east. In Bohemia the weather was somewhat clear to cloudy, in Moravia almost clear weather prevailed. Afternoon maximum temperatures in Bohemia reached 24–28 �C, in Moravia 26–30 �C. Very mild local rainfalls occurred in Bohemia; their daily totals did not exceed 0.3 mm.

The highest NO₂ concentration (193 μg.m^-3) was measured in Liberec and the levels above 100 μg.m^-3 were registered also at monitoring stations Ostrava-Fifejdy, Prague-Smíchov and – which was to a certain extent striking – also at the mountainous station Rudolice v Horách where in early morning hours the impact of smoke plume from the power plant had apparent influence. The lowest value of 3 μg.m^-3 was recorded in Churáňov. Concentrations between 40 and 98 μg.m^-3 were recorded in larger cities, the range between 20 and 30 μg.m^-3 was characteristic for small towns, and the values below 12 μg.m^-3 were reported by the stations situated in higher altitudes. The highest ozone concentration 202 μg.m^-3 was measured in Karviná, the lowest value 105 μg.m^-3 in Sokolov. The spatial distribution of ozone concentrations shows a marked difference between the south-western and north-eastern parts of the Czech Republic. In the Šumava Mts. and in the Bohemian Forest Mts. the concentrations ranged around 110 μg.m^-3, while in north-eastern Bohemia and Moravia they reached 196–202 μg.m^-3. The above mentioned distribution resulted from the fact that beginning from 21 August colder air mass was entering Bohemia behind the cold front, while in the eastern part of the country�s territory the exchange of the original air mass had not occurred yet. (Fig. 2.4.4)

Fig. 2.4.1 Course of maximum daily temperatures, their long-term average and daily total precipitation at Prague-Libuš station, occurrence of ozone concentrations exceeding 180 μg.m^-3 on the territory of the Czech Republic 1.6.2002–15.9.2002

Fig. 2.4.2 Spatial distribution of maximum hourly concentrations of NO₂ a O₃, 10. 7. 2002

Fig. 2.4.3 Distribution of maximum hourly ozone concentrations on the territory of the Czech Republic, 10 July 2002, and three-hour average ozone concentrations in Austria, 10 July 2002 at 16:00 CEST.

Fig. 2.4.4 Spatial distribution of maximum hourly concentrations of NO₂ a O₃, 23. 8.2002