The first two and the last two months of the year 2006 had significant differences as concerns temperatures. The average monthly temperature in January 2006 was –6.0 �C, i.e. markedly subnormal, with the deviation from the normal –3.2 �C; February temperatures were also slightly subnormal, with average temperature –3.0 �C and deviation –1.9 �C. On the contrary, November with the average monthly temperature 5.6 �C and the deviation +2.9�C and December 2006 with the temperature 2.3 �C and the deviation +3.3 �C, were markedly above normal. As for precipitation, the winter months were markedly fluctuating. The highest precipitation was recorded in February 2006 with the total of 44 mm, which represents 116 % of the long-term normal. The remaining months were subnormal: in November 2006 46 mm were recorded, i.e. 93 % of the normal, in December 2006 28 mm, i.e. 59 % of the normal, and in January 2006 35 mm, i.e. 34 % of the normal.

In the periods January–February and November–December 2006 anticyclonic situations occurred in 43.3 % cases and their relative frequency for the given months was considerably uneven and ranged between 25.0 and 61.2 %. The biggest relative frequency of anticyclonic situations (61.2 %) was detected in January 2006 and the lowest relative frequency 25.0 % occurred in February 2006. In November and December 2006 the frequency was 36.7 % and 48.4 % respectively. The occurrence of anticyclonic situations in January 2006 with the total of 19 days is the third highest since the year 1946. Higher frequency occurred only in 1964, and namely 26 days, and in1946 – 25 days.

The cause of markedly subnormal temperatures in January 2006 was the influx of very cold air from northeast in the last decade around the anticyclone with its centre above northeast Europe. In the period of 22–24 January the minimum temperatures dropped to the values from –15 to –20 �C in Bohemia, and from –20 to –25 �C, occasionally to –30 �C in Moravia. Only during the first and fourth pentade warmer oceanic air flowed to central Europe and maximum temperatures on 20–21 January reached from 1 to 4 �C, occasionally up to 8 �C. The temperatures in the last two months of the year 2006 were markedly above normal, because in the period from 15 November to 19 December southwest cyclonic and anticyclonic situations prevailed which brought warmer air from south latitudes (Fig. II.5.1).

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 January and February 2006 3 localities recorded SO₂ concentrations exceeding 250 μg.m^-3 for 3–7 hours: station Krupka with the maximum value of 273 μg.m^-3, Teplice 331 μg.m^-3 and Horní Halže 1094 μg.m^-3. The analysis of the changes in wind direction suggests that there was the impact of the plume from a large source onto earth surface. Similarly, in December 2006 the SO₂ concentrations at the station Komáří Vížka exceeded 250 μg.m^-3 for 3 hours and the maximum value was 376 μg.m^-3. In the monitored period the NO₂ concentrations exceeding 200 μg.m^-3 for 3–7 hours were recorded at two stations: Plzeň-Slovany and Prague-Legerova. On 1 and 3 February 2006 increased concentrations of NO₂ were recorded at the station Plzeň-Slovany for 6 and 4 hours respectively, with the highest value of 264 μg.m^-3. On 24 November and on 7 and 15 December increased concentrations were recorded at the station Prague-Legerova for 3–5 hours and the maximum value (252 μg.m^-3) was reached on 15 December 2006.

Isolated exceedances 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 1.10.2006 to 31.12.2006 are depicted in Fig. II.5.2.

As for maximum daily temperatures at the station Prague-Libuš, the period from 1.4. to 30.9.2006 was slightly above normal, with the deviation from the normal +1.9 �C, but the temperatures were significantly variable. Temperature deviations from the normal ranged in the interval from +12.6 �C to –10.1 �C. The average monthly temperature deviations showed the greatest difference between July with the deviation +6.0 �C and August with the deviation of –2.4 �C. The monitored period recorded 25 days with the temperature above 30 �C and the highest value 35.1 �C was measured on 27.7.2006 (Fig. II.5.3).
As for the occurrence of increased concentrations it can be stated that meteorological conditions for the creation of ground-level ozone were very favourable, especially in July, with concentrations exceeding 200 μg.m^-3on 5 days; the maximum value (241 μg.m^-3) was measured at the station Lom on 19 July 2006. On 6 days of July – in three subsequent hours – concentrations exceeding 180 μg.m^-3 were measured, thus the alert threshold limit value for ground-level ozone for the information of public was exceeded. The overview of conditions for issuing the signal follows below:

• 19.7.2006 6 stations with maximum value of 241 μg.m^-3 at the station Lom
• 20.7.2006 13 stations with maximum value of 220 μg.m^-3 at the station Lom
• 21.7.2006 15 stations with maximum value of 227 μg.m^-3 at the station Prague 6-Suchdol
• 26.7.2006 3 stations with maximum value of 195 μg.m^-3 at the station Karviná
• 27.7.2006 15 stations with maximum value of 221 μg.m^-3 at the station Rudolice v Horách
• 28.7.2006 5 stations with maximum value of 224 μg.m^-3 at the station Prague 6-Suchdol

On 19 and 20 July and for the major part of 21 July the ridge of anticyclone of 1023 hPa with the centre above the North Sea and later above the Norwegian Sea reached the territory of the Czech Republic; in its rear very warm air flew to central Europe. During the late afternoon hours on 21 July weak cold front passed over Bohemia towards the east. On 22 July this front passed over to Moravia and separate temporary anticyclone of 1020 hPa was created above the territory of the Czech Republic, which disintegrated on the following day and another markedly cold front came from the west.

On 19 and 20 July and for the most part of 21 July the weather was clear to somewhat cloudy with maximum temperatures from 31–36 �C to 32–37 �C. Precipitation occurred only in northwest Bohemia with daily totals to 0.3 mm. On 21 July in the late afternoon hours the sky was cloudy to overcast with showers during which the total precipitation amounted to 1 mm, sporadically up to 9–13 mm. Maximum daily temperatures amounted to 33–37 �C. On 22 July the weather was mostly somewhat clear, in northwest Bohemia cloudy with precipitation up to 5 mm. The maximum temperatures reached 32–35 �C.

The anticyclone of 1023 hPa with the centre above the North Sea was the leading formation of the meteorological situation in the period of 26–28 July 2006. It was slowly moving to the north to the area of the Spitsbergen and reached the area of the Mediterranean Sea by the ridge of the anticyclone. In its rear the influx of very warm air culminated over the territory of the Czech Republic. In the afternoon on 28 July cold front spread into Bohemia and during the night to Moravia, accompanied by showers and wind gusts.
On 26 and 27 July 2006 and for the most part of 28 July the weather was somewhat clear with increasing temperatures from 31–35 �C to 33–37 �C. In the afternoon hours of 28 July the weather in the Czech Republic was cloudy, temporarily cloudy with showers; the precipitation totals reached up to 20 mm in the west and northwest, in the mountainous areas occasionally up to 42 mm. On the following day the weather was cloudy with showers and the daily totals of 2–10 mm, in the mountainous areas occasionally up to 40–60 mm; the maximum temperatures amounted to 26–30 �C in Bohemia and 28–34 �C in Moravia.

In 2006 the concentrations of the ground-level ozone exceeding 180 μg.m^-3 were recorded as early as in March, nevertheless these were occasional occurrences lasting less than 3 hours. In May and June the alert threshold limit value for the ground-level ozone was exceeded on 9 and 6 days, respectively, nevertheless these were short-term exceedances, i.e. less than 3 hours. In July the alert threshold limit value was exceeded in total on 14 days, and the exceedances of the alert threshold limit value lasting 3 hours and more were recorded on the following days: 19–21 July and 26–28 July 2006. In August and September 2006 the alert threshold limit value was not exceeded in the territory of the Czech Republic.

Within the smog and warning regulation system the possibility to issue the information for the public, based on the prognosis of the smog situation, was used. For the territory of Bohemia the signal was issued on 13 July 2006 and for the whole territory of the Czech Republic on 19 and 20 July (the signal was prolonged up to 29 July). In the first case the alert threshold limit value for the ground-level ozone was exceeded only at the station Rudolice v Horách (the measured concentration was 191 μg.m^-3). On 19–21 July and 26–28 July 2006, frequent exceedances of the limit value occurred and the signal was cancelled on 29 July 2006. On 22–24 July 2006 occasional exceedances of the value 180 μg.m^-3 were recorded, but only for less than 3 hours. Figs. II.5.4 and II.5.5 show the spatial distribution of maximum hourly concentrations of ground-level ozone on 21 and 27 July 2006. In the first case the belt of the highest concentrations was stretching over central Bohemia from the south to the north and the highest values were detected north of Prague. On 27 July the highest concentrations occurred north of Plzeň and the secondary maximum is apparent north of Prague.

Fig. II.5.1 Air temperature deviation from the long-term normal, Prague-Libuš, 1.1.–28.2.2006 and 1.11.–31.12.2006

Fig. II.5.2 Average hourly NO₂ concentrations at the station Prague 2-Legerova, 1.10.2006–31.12.2006

Fig. II.5.3 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, 1.4.2006–30.9.2006

Fig. II.5.4 Spatial distribution of maximum hourly concentrations of ground-level ozone on the territory of the Czech Republic, 21.7.2006

Fig. II.5.5 Spatial distribution of maximum hourly concentrations of ground-level ozone on the territory of the Czech Republic, 27.7.2006