January and February 2005 had marked differences as concerns temperatures. The average monthly temperature in the Czech Republic in January was –0.4 �C, with the deviation +2.4 �C, the February average temperature reached –3.7 �C with the deviation –2.6 �C. November and December 2005 recorded steady temperatures in the long-term. The average monthly temperature in November was 2.1 �C, with the deviation –0.4 �C, in December –1.1 �C, with the deviation –0.4 �C. As for precipitation, January and February 2005 were significantly above normal with monthly total precipitation 69 mm and 64 mm respectively, which represents 165 % and 170 % of the long-term normal. November and December 2005 were significantly different as concerns precipitation. Monthly total precipitation in November 2005 was 33 mm, which is 67 % of the long-term normal, in December 2005 70 mm, which is 152 % of the long-term normal.

Anticyclonic situations in January–February and November–December 2005 occurred in 23 % of cases and their frequency for the mentioned months was quite even and ranged between 6 and 9 cases per one month. The reason of extremely high temperatures in the first half of January 2005 was strong zonal circulation in the air connected with the ridge of anticyclone, which reached over central Europe for the most of January and February. The territory of the Czech Republic was thus influenced by warm sectors of cyclones and southern edges of frontal disturbances, which moved to the east along the northern edge of the mentioned ridge. Maximum daily temperature on 8 January at the station Prague-Libuš reached 14.0 �C and exceeded the long-term average by 12.9 �C. Colder period lasting from 23.1. to 30.1.2005 brought the day with minimum temperature –8.7 �C (27.1.2005) at the station Prague-Libuš, and in the period 4.2.–9.2.2005, the minimum temperature decreased to –12.7 �C (7.2.2005).

In spite of meteorological situations with unfavourable dispersion conditions, as potential conditions for increasing pollutants� concentrations, smog situation was not created and no signals were issued. In February 2005 3 localities recorded, for the period of 3–4 hours SO2 concentrations higher than 250 μg.m^-3, and namely the stations Ostrava-Zábřeh with maximum value of 615 μg.m^-3, Krupka 349 μg.m^-3 and Komáří Vížka 458 μg.m^-3. At the station Prague-Legerova there were measured NO2 concentrations exceeding 250 in 7 days in February and January, for the period of 3–7 hours with the highest value of 349 μg.m^-3. Isolated exceedences occurred mainly at one station only, and pollutants� concentrations decreased quickly in all cases; thus the conditions for issuing the signals of smog and warning regulation system were not fulfilled. The changes of hourly averages of NO2 concentrations at the monitoring station Legerova in the period from 1.1.2005 to 28.2.2005 are shown in Fig. II.7.1.

As for maximum daily temperatures at the station Prague-Libuš, the warm period from 1.4. to 30.9.2005 was slightly above normal, with the deviation from the normal + 1.1 �C, but the temperatures were markedly variable. Temperature deviations from the normal ranged in the interval from +12.6 �C to –11.3 �C, with the highest frequency in the first half of the monitored period. The temperatures exceeded 30 �C on 9 days; the highest value 36.5 �C was measured on 29.7.2005 (Fig. II.7.2).

As for the occurrence of increased concentrations it can be stated that in spite of the fact that meteorological conditions for the creation of ground-level ozone were not significantly favourable, concentrations exceeding 200 μg.m^-3 occurred on two days. Concentrations exceeding the alert threshold limit value for ground –level ozone 180 μg.m^-3 were recorded on 7 days during the period 1.4.–15.9.2005, and namely on 28.–30.5. 2005, 25.6.2005, 15.7.2005 and 28.–29.7.2005. On 3 days the alert threshold limit was exceeded at one station only, on 2 days at 2 stations, on 1 day at 3 and 7 stations. The total time of the exceedence of the alert threshold limit for ground-level ozone during the daytime was 1–3 hours (3 cases), 4 hours (2 cases) and 7 hours (one case). The continuous exceedence of the alert threshold lasting for 4 hours and longer was recorded in total in 2 cases: 29.5.2005 at the station Prague-Libuš for 4 hours and with maximum value 214 μg.m^-3, 29.7.2005 at the station Rudolice v Horách for 7 hours and with maximum value 235 μg.m^-3. More detailed description of meteorological conditions and changes of ground-level ozone concentrations is presented for these two cases.

On 28.5.2005 the centre of anticyclone of 1020 hPa was situated above the Ukraine, on the following day above Slovakia and the centre of the cyclone of 1000 hPa above Ireland, on the following day above western Europe. Between these two formations warm air from the south flowed over the territory of the Czech Republic. Both on 28 and 29 May the weather in the Czech Republic was mainly clear, maximum afternoon temperatures on 28.5. reached 28–32 �C, on the following day 29–34 �C. The wind, at first blowing from the south-east, later irregularly also from the north, had the velocity to 4 m.s-1. On 30.5.2005 cold front was moving across Bohemia in the noon hours, in the afternoon and during the night across Moravia and Silesia, accompanied by weak showers in western Bohemia, in other territories by slight and strong showers. The local precipitation reached up to 39 mm. Weak wind from the south blew in front of the front, behind the front the wind was blowing from north-west. The temperatures dropped considerably behind the cold front and on 31.5.2005 the afternoon maximum temperatures ranged only between 15–19 �C.

Since 25.5.2005 the ground-level ozone concentrations were gradually increasing from the start value of about 120 μg.m^-3, and on 28.5. they reached 187 μg.m^-3 (Kladno) and on 29.5.2005 214 μg.m^-3 (Prague-Libuš). The alert threshold limit value exceedence at the Prague-Libuš station lasted for 4 hours from 13:00 to 17:00 of summer CET (Fig. II.7.3). On the following day ozone concentrations decreased to 150–165 μg.m^-3, only at the stations Litoměřice and Mikulov-Sedlec the short-time exceedence of 180 μg.m^-3 was recorded.

On 28.7.2005 the circulation above central Europe was influenced by the anticyclone of 1020 hPa with the centre above the Black Sea and the cyclone of 1005 hPa above the Bay of Biscay. Warm air flowed above the territory of the Czech Republic between these formations. On the following two days insignificant anticyclone remained above eastern Europe and the cyclone moved over southern England as far as to Denmark. At its rear cold front moved to the east. In front of the cold front, on 29 July, the influx of very warm air from the south culminated. In the evening hours the cold front moved across Bohemia and on the following day across Moravia and Silesia. On 28 July the weather was largely almost clear with maximum temperatures from 31 to 36 �C, rainfall occurred in isolated cases and reached max. 0.7 mm. On the following day the temperatures in Bohemia increased to 33–37 �C, in Moravia to 32–36 �C. During the night the cold front brought rainfall, in western Bohemia up to 26 mm, in other localities in Bohemia 2–11 mm, in northern Moravia up to 1 mm. On 30.7.2005 behind the cold front the weather was cloudy with showers and the temperatures during the day reached 26–30 �C, in eastern Moravia locally still 34 �C.
Since 27.7.2005 concentrations of ground-level ozone had been quickly increasing from the value of about 115 μg.m^-3 up to 235 μg.m^-3 (29.7.2005 at the station Rudolice v Horách). On the given day the alert threshold limit value limit 180 μg.m^-3 was exceeded in the total at 7 stations in northern Bohemia, out of which at the station Tušimice for 4 hours in the total, but not continuously. The continuous period of alert limit value exceedence in the duration of 7 hours was recorded at the station Rudolice v Horách from 12:00 to 18:00 of summer CET (Fig. 3). On the following day the ozone concentrations in Bohemia decreased to 110–130 μg.m^-3, in Moravia to 130–150 μg.m^-3.

Within the smog and warning regulation system the possibility to issue the information for the public, based on the prognosis of smog situation, was used. Information valid for the whole territory of the Czech Republic was issued for the days 28.–29.7.2005 and 1.–2.9.2005. In the first case the prognosis came true and the smog situation occurred, in the second case the maximum concentrations reached only 167 μg.m^-3 and the alert threshold limit was not exceeded.

The system of advanced information for the public, enabled by the provision of regulation rules giving the possibility to issue the information already in case of the prognosticated smog situations, without waiting for the exceedence of the limit value, compensates for the inevitable risk of �false alarm�. This approach is preferred also due to the fact that, after issuing such information, there is no need of demanding organizational measures connected with economic impacts.

Fig. II.7.1 Average hourly NO₂ concentrations at the station Prague 2-Legerova, 1.1.2005–28.2.2005

Fig. II.7.2 Course of maximum daily temperatures, their normal and daily total precipitation at Prague-Libuš station. Occurrence of ozone concentrations exceeding 180 μg.m^-3 on the territory of the Czech Republic, 1.4.2005–30.9.2005

Fig. II.7.3 Spatial distribution of maximum hourly concentrations of ground-level ozone on the territory of the Czech Republic, 29.7.2005