Both January and February of the year 2000 were slightly above normal in their average temperatures, while February with the deviation +3,6 �C was the coldest month of the 1999/2000 winter period. Precipitation in January and February was slightly higher than the long-term normal (January 126 % and February 118 % of the normal). In higher altitudes the snowfall ranged from 60 to 200 cm, and there were considerable local differences in the height of the snow cover. In the Jeseníky Mts., with up to 230 cm, the continuous snow cover lasted until May 2000.

In central Europe the mixed to zonal circulation prevailed in the winter period.

Consequently, the frontal systems passed over central Europe from west or northwest. Occasionally the Czech Republic�s territory was influenced by a ridge of higher pressure for one to two days, the whole winter period in the beginning of 2000 did not record any several-day stationary anticyclone in central Europe.

The development of dispersion conditions in the beginning of the year was determined by the above mentioned character of meteorological situation. Due to relatively quick air flows the temperature inversions occurred near the earth surface usually for a part of day only, especially in the early morning, and disappeared during the afternoon. In cases of day-long inversion the inversion was elevated, and thus sufficient wind velocity and good ventilation were maintained. During the winter period in the beginning of the year 2000 no day with unfavourable dispersion conditions was recorded. This situation occurred for the first time since 1988. The days with moderately unfavourable dispersion conditions lasting for 24 hours, were recorded mainly in the first half of January 2000. This period of the winter season was assessed as extraordinary favourable.

In the end of the year 2000, represented by November and December, the trend of temperature deviations slightly above normal recorded in the beginning of the year, continued. November 2000 was the warmest month within more than 25 years with the deviation +2.9 �C from the normal. The temperature normal of December was exceeded by +1.6 �C. Precipitation on the most territory of the Czech Republic was subnormal both in November and December, and especially in December with total precipitation of 59 % of the monthly normal in Bohemia. In Moravia the precipitation was higher: November 2000 had 135 % of the long-term average. This influenced also the amount of snow in the frontier mountains where the height of snow cover partly increased as late as in January and February 2001.

After the longer period without activating the Smog Regulation System (SRS) the warning signals were issued in Prague and Ostrava in the end of 2000. In other smog areas of the Czech Republic there was no need to issue the signal. The warning signals in Prague were issued with regard to the occurrence of increased NO_x concentrations near communications, and mainly in the periods of the influence of high pressure above central Bohemia. The warning signal in Prague was issued for 6 days in total during November and December. In the Ostrava Region the warning was issued due to the increased PM₁₀ concentrations. One-day episode (21. 12. 2000) was the result of inversion stratification of the atmosphere due to the warm air advection from southeast around the anticyclone passing from Poland to Ukraine.

The fact that during the recent 4 winter periods it was not necessary to issue the regulation of stationary air pollution sources within the SRS shows that the measures aimed at emission reduction from combustion processes in the Czech Republic are efficient. Further development of SRSs and their application will be determined by the way of EU Directives implementation.

In summer 2000 significant episode of summer photochemical smog was recorded after one-year pause, and namely on 19–22 June. Another period with increased ground-level ozone concentrations exceeding the alert threshold of 180 μg.m^-3 at 24 monitoring stations occurred between 13 and 21 August 2000. The both periods were characterized by hot summer weather with maximum daily temperatures measured at Prague-Libuš meteorological station exceeding 30 �C. The summer character of the weather was typical mainly for the June episode.

Between 18 and 22 June 2000 the anticyclone moved from the Azores to the central Europe and became the leading pressure formation for the prevailing part of the European continent. Depressions and the connected frontal systems circled around the leading anticyclone and were moving from the eastern part of the Atlantic over Iceland, Norway Sea and Barents Sea towards the northern Urals and the Volga river basin. The flow of cold air to the Czech Republic, prevailing in the beginning of the studied period, was replaced by the advection of warm air on 19 June. It�s culmination in Bohemia was recorded on 21 June, and in Moravia and Silesia on 22 June 2000. On 18 June 2000 the whole territory of the Czech Republic had somewhat cloudy weather and only occasionally immeasurable precipitation occurred. Maximum afternoon temperatures increased to 26–27 �C. On 19, 20 and on the prevailing part of 21 June the whole territory of the Czech Republic was almost clear and afternoon maximum temperatures increased gradually from 30 �C up to 37 �C.

Further course of the leading anticyclone aimed towards the Ukraine over the Carpathian Mts. and in the end of the studied period the anticyclone was located above Sicily. During the passage this anticyclone weakened and along its northern edge weak cold front entered Bohemia on 21 June 2000. The maximum temperatures in Bohemia amounted to 35 �C, while in Moravia and Silesia very hot weather with maximum 38 �C was maintained. The ozone episode was ended up by further cold front which passed over the territory of the Czech Republic on 22 June in late afternoon hours.

Fig. 2-99 shows the time-height cross-section of air temperature and of temperature gradients derived from the aerological measurements carried out by the Prague-Libuš Observatory. Afternoon temperatures increased from 22 �C to 32 �C and in the end of the studied period decreased to 22 �C. During the night hours from 18 to 22 June 2000 ground- level air temperature inversions were recorded up to 500–800 m above sea level, i. e. 200–500 m above the ground.

Deteriorated dispersion conditions in the anticyclone resulted in the increase of maximum average hour NO₂ concentrations between 18 and 22 June 2000. On 18 June the majority of measuring stations recorded NO₂ concentrations ranging from 5 to 20 μg.m^-3, occasionally 40 μg.m^-3, on 20–22 June 10–60 μg.m^-3 on the whole territory of the Czech Republic, at urban stations 80–96 μg.m^-3. After the passage of the cold front the NO₂ concentrations decreased to similar values measured in the beginning of the studied period.

On 18 June 2000, maximum average hour ozone concentrations at most measuring stations ranged between 100 and 200 μg.m^-3. Sporadic values about 130 μg.m^-3 occurred only in Bohemia. On the following day the concentrations increased by 20–30 μg.m^-3 on the whole territory of the Czech Republic and at five stations exceeded the value 180 μg.m^-3. On 20 June further increase was recorded and at most stations the ozone concentrations reached 150–170 μg.m^-3. The number of measuring sites, at which the alert threshold of 180 μg.m^-3 was exceeded, amounted to 6 and in Pardubice the maximum value of 207 μg.m^-3 was recorded. On 21 June the limit value 180 μg.m^-3 was exceeded at 20 stations in Bohemia, which represents 39 % of all monitoring stations in the Czech Republic. At 9 stations the ozone concentrations exceeded 200 μg.m^-3 and the highest value of 225 μg.m^-3 was recorded in Pardubice.

On the following day the ozone concentrations reached the levels over 180 μg.m^-3 at 23 stations, i.e. at 45 % of the measuring sites in the Czech Republic. At the Pardubice-Rosice station the average hourly concentration of 256 μg.m^-3 was recorded which is the highest value reached during the whole monitored episode. After the passage of the cold front the air mass was changed and maximum ozone concentrations at numerous urban stations reached only 20–60 μg.m^-3 on the prevailing part of the territory of the Czech Republic 60–120 μg.m^-3, and only in southeast and northeast Moravia the concentrations still reached 60 μg.m^-3. The development of the field of ground-level ozone concentrations on the territory of the Czech Republic during the described ozone episode is depicted in Fig. 2-100.