The first two and the last two winter months of the year 2007 had significant differences as concerns temperatures. The average monthly temperature in January 2007 was +3.2 �C, i.e. markedly above the normal, with the deviation from the normal +6.0 �C; similar situation was in February 2007 with the average temperature +2.8 �C and the deviation +3.9 �C. On the contrary, November had the average monthly temperature +1.4 �C and the deviation from the normal −1.3 �C; December 2007 with the average temperature −0.9 �C differed from the normal only by +0.1 �C. The average daily air temperatures in the period 1.1.–28.2.2007 and the long-term normal at the station Prague-Libuš are presented in Fig. II.5.1. As for precipitation, January and November were markedly above the normal with monthly totals 75 mm and 76 mm, which represents 180.7 and 153.8 % of the long-term normal respectively. In February 2007 the precipitation reached 116.4 % of the normal and a markedly subnormal level was recorded in December 2007 with monthly total of 35 mm, which is 73.4 % of the normal.

In the periods January–February and November–December 2007 anticyclonic situations occurred in 25.0 % of days and their relative frequency for the given months was considerably uneven and ranged between 9.1 and 38.7 %. The biggest relative frequency of anticyclonic situations (38.7 %) was detected in December 2007 and the lowest relative frequency (9.7 %) was in January 2007. In February and November the relative frequency reached 21.4 % and 30.0 % respectively.

The cause of markedly above-the-normal temperatures in January 2007 was the prevailing cyclonic character of the weather with western zonal circulation bringing warmer and moister oceanic air above the territory of the Czech Republic. For the prevailing part of the period the centres of the cyclones moved over northern England above central Scandinavia and towards the area of St. Peterburg, only on 19 January 2007 the centre of the cyclone 965 hPa moved over Denmark and the territory of the Czech Republic was stricken by hurricane Kyrill. Due to the quick exchange of air masses there were good dispersion conditions on the territory of the Czech Republic for almost whole January 2007. The worst dispersion conditions were in December 2007.

In spite of the meteorological situations with unfavourable dispersion conditions, as potential conditions for increasing pollutants concentrations, smog situation was not created and no signals were issued. In December 2007 three localities in the Ústí nad Labem Region recorded SO₂ concentrations exceeding 250 μg.m^-3 for 3–21 hours: on 20.12. 2007 at the station Most for 3 hours, at the station Lom for 5 hours and at the station Teplice for 10 hours with the maximum hourly average 398 μg.m^-3, on 21.12. 2007 at the station Most for 7 hours, at the station Lom for 17 hours and at the station Teplice for 21 hours with the maximum value of 554 μg.m^-3. The concentrations of NO₂, exceeding 200 μg.m^-3, were recorded only at the station Prague 2-Legerova. The changes of hourly averages of NO₂ concentrations at the monitoring station Legerova from 1.1. to 28.2.2007 and from 1.11. to 31.12.2007 are depicted in Figs. II.5.2 and II.5.3. Both in SO₂ and in NO₂ there occurred exceedances of the set limit value, but only in short terms and the concentrations of the pollutants declined in all cases very quickly. Consequently, there were not fulfilled the conditions for issuing any signals.

With regard to the maximum daily temperatures at the station Prague-Libuš, the period from 1.4. to 30.9.2007 was slightly above normal with the deviation from the normal +2.1 �C, but the temperatures were very fluctuating. The temperature deviations from the normal ranged between +13.6 �C and –9.1 �C. The average monthly temperature deviations in the monitored period recorded irregularly declining tendency: in April 2007 the deviation was +5.2 �C and in September 2007 –1.1 �C (between May and June there was an increasing trend). In the monitored period there occurred 16 days with the temperature above 30 �C and the highest temperature (37.6 �C) was measured on 16.7.2007.

Meteorological conditions for the creation of the ground-level ozone were favourable beginning from the last decade of the month of May to the end of the second decade of August. The most favourable conditions were in July when on 5 subsequent days there occurred concentrations above 180 μg.m^-3; on 16.7.2007 the station Rudolice v Horách measured the concentration 240 μg.m^-3. Maximum daily air temperatures at the station Prague-Libuš and the occurrence of ground-level ozone concentrations above 180 μg.m^-3 on the territory of the Czech Republic in the period 14–22 July 2007 are shown in Fig. II.5.4. The spatial distribution of the daily maximum hourly ground-level ozone concentrations on the territory of the Czech Republic from 14 to 22 July 2007 is shown in Fig. II.5.5.

From 15 to 21 July 2007 an anticyclone stayed over the south-eastern Europe; in its rear, very warm air flowed to central Europe for the most part of the period. The cyclones moved form the eastern part of the Atlantic Ocean across the British Isles and Scandinavia over the Barents Sea. In the rear of the cyclones the undulated cold fronts shifted over western to central Europe. The first one moved over Bohemia on 18 July and on 19 July over Moravia to Slovakia. In front of the second cold front, which was crossing over central Europe on 21.7., warm air came from southwest to the territory of the Czech Republic on 19 July and 20 July. This undulated cold front was followed by quiet a long period with marked temperature drop.

On 15–17 July 2007 the weather was prevailingly clear, in the north-western Bohemia temporarily somewhat clear and daily maximum temperatures reached from 32 to 38 �C. On 18 July the weather in Bohemia was cloudy, temporarily up to overcast with showers and local storms. In Moravia and in Silesia the weather was clear to somewhat clear. The maximum daily temperatures in Bohemia amounted to 25–29 �C, in Moravia to 27–32 �C, locally up to 37 �C. The daily total precipitations in Bohemia were 1–3 mm, locally up to 37 mm. In Moravia the precipitations were isolated and did not exceed 4 mm. On 19 July the weather in Bohemia was cloudy, temporarily somewhat clear with great differences in daily precipitation totals. On the prevailing part of the territory there were recorded precipitation totals from non-measurable amount below 7 mm up to 28 mm in the west. In Moravia and Silesia the precipitations were not measurable in most cases. Maximum daily temperatures in Bohemia amounted to 28–32 �C, in Moravia to 29–33 �C, occasionally up to 37 �C. On 20 and 21 July the weather in Bohemia was somewhat cloudy, with precipitations on the most part of the territory, with great local differences. Higher precipitation totals were recorded on 20.7., reaching occasionally up to 47 mm in Moravia, and up to 22 mm in Bohemia. On the following day the precipitations were higher in Bohemia, with the totals occasionally 34 mm, in Moravia there was no precipitation on the most of the territory and in places of occurrence it did not exceed the value of 1 mm.  Afternoon maximum temperatures on 20 July reached 31–35 �C in Bohemia,  in Moravia 32–37 �C. On the following day the maximum temperatures on the whole territory dropped by 6–8 �C. In the period from 1 April to 30 September 2007 there occurred, according to the Prague-Libuš station records, 5 periods with the afternoon temperatures higher than 27 �C for 3 and more days. During high air temperatures and clear or somewhat clear weather the increased concentrations of ground-level ozone can be expected.

Table II.5.2 shows quite clearly that the highest concentrations were measured on the days, when there were the highest afternoon air temperatures. From 16 to 20 July 2007 the alert threshold limit value 180 μg.m^-3 was exceeded. The greatest number of stations exceeding the limit value was registered on 17.7.2007 (see Tab. II.5.1). On 16–17 and 20 July the value of 180 μg.m^-3 was exceeded in Bohemia and on 17–20 July in Moravia. Within the smog and warning regulation system the signal was issued for all regions in the Czech Republic on 17.7.2007. The signal was cancelled on 18 July 2007 in mid-morning hours. Similarly, in the other two periods with high afternoon temperatures, the maximum concentrations almost reached the alert threshold limit for ground-level ozone, i.e. 180 μg.m^-3. Due to the fact that ozone creation is influenced, besides sun radiation, by a number of other factors, the relation to temperature is looser as results from the given remaining two cases.

Tab. II.5.1 The overview of concentrations O₃ above 180 μg.m^-3 lasting 3 hours and more

Tab. II.5.2 Overview of the periods with air temperatures above 27 �C for 3 or more days, and of the respective measured maximum ground-level ozone concentrations

Fig. II.5.1 Course of daily average air temperatures and the long-term normal, Prague-Libuš, 1.1.–28.2.2007


Fig. II.5.2 Average hourly NO₂ concentrations at the station Prague 2-Legerova, 1.1.–28.2.2007


Fig. II.5.3 Average hourly NO₂ concentrations at the station Prague 2-Legerova, 1.11.–31.12.2007


Fig. II.5.4 Course of maximum daily air temperatures, their normal and daily sums of precipitation at the station Prague-Libuš. Occurrence of ozone concentrations exceeding 180 μg.m^-3 in the territory of the Czech Republic, 1.4.–30.9.2007


Fig. II.5.5 Spatial distribution of maximum daily hourly concentrations of ground-level ozone in the territory of the Czech Republic, 14.7.–22.7.2007