The Ignalina NPP is situated in the temperate climate zone. The region concerned, as well as all Lithuanian territory, is located along the path dominated by western wind currents, therefore in the global sense its climate can be considered as homogeneous. However, on the regional scale it is rather variable, because of the prevalent intrusion of air flows from the adjacent geographical zones [8].
The territory of the Lithuanian Republic is divided into four climatic regions, depending on their proximity to the Baltic see, the orography of relief and the diversity of the underlying surface. The territory of concern belongs to the East climatic subarea [9]. In comparison with other Lithuanian areas, this area is marked by a big variation of air temperature over the year, the colder and longer winters with abundant snow cover, and warmer, but shorter summers.
On the whole the local climate depends on the circulation of air mass from the Atlantic, but the influence of air mass from the continents of Europe and Asia continent are perceptible as well [10].
Wind Regime
About 60 cyclones and 50 anticyclones are expected yearly due to the weather conditions of the territory concerned. Cyclones are influenced by the weather about 170 days and anticyclones about 130 days a year, because they are moving faster. During the rest of the time baric formation are observed [8].
The entire territory of Lithuania has practically no influence on the formation of new air masses or their considerable transformation. During the year about 170 atmospheric fronts pass over the Ignalina territory.
During the cold season the warm fronts predominate over the cold ones, while during the warm seasons they are distributed equally. Colds fronts move faster than the warm ones. Western and southern winds predominate. The strongest winds have western and south - east directions. The average annual wind speed is 3.5 m/s, and maximal (gust) speeds can reach 28 m/s. No-wind conditions are observed on the average of 6 % of the time and last no more than one day (24 hours) in the summer, and no more than two days in the winter [8].
The predominant wind direction changes depending on the distance above the ground. Beginning from the 200 m distance above ground the predominate direction is as follows: in January from south to south-west, in April from south-south-east to south-east, in October from west-north-west to north. Only during July is the predominate direction west at elevated altitudes.
The wind velocity changes depending on the distance from the ground surface. At a distance of 100 m from the ground the average wind velocity doubles in comparison with wind velocities at the height of the wind vane. They continue to grow within the half km layer. Then the increase of the wind velocity as a function of height decrease. On the whole the area atmospheric conditions are favorable for scattering substances from the plant ventilating stacks.
Hurricanes and Spouts [12,13]
Spouts in the vicinity of the Ignalina NPP do not exceed class F-2 according to Fujita classification [11]. The probability of a class F-2 spout for the plant platform with one km2 area is one time in more than 61667 years. For a class F-1 spout such a probability is one time per 43023 years. As a calculated characteristic of a spout for the Ignalina NPP platform with one km2 area is assumed to be the characteristic of a class F-0 spout, the probability of which is one time per more than 10000 years.
The season of spouts begins at the end of April and ends in the first half of September. The directions of spout motion is from south-west to north-east in 73 % of the cases. The average length of spout shift trajectory is 20 km and the length varies from 1 to 50 km. Average width of the spouts is 50 m, and it varies mostly from 10 to 300 m. Calculated maximal spout velocity with frequency one time per 10000 years is about 39 m/s.
Data about the most destructive spouts are incomplete. However, the following data in normally used for calculations:
Sunshine and Cloudiness
Average annual duration of sunshine in the region is about 1710 hours (42 % of the maximum possible duration of the earth's surface irradiation by the sun). June is the most sunny month: the amount of sunshine in June is about 280 hours (58 % of the possible duration). The shortest period of sunshine because of cloudy weather is observed in December, which is about 20 hours (12 % of possible duration) [14].
Average annual cloudiness in region is about a force 7, and in December it increase to a force 8.5 and in May it decrease to a force 6.5. The average annual amount of cloudy days (175) is considerably larger than the clear ones.
Air Temperature
Average annual air temperature in the region is 5.5 oC. January is the coldest month with an average monthly temperature of - 6.5 oC, and June is the warmest one with 17.8 oC. Annual amplitude of average monthly temperatures is 24.1 degrees. Absolute maximum of recorded temperature is 36 oC, and absolute minimum is - 40 oC. The greatest oscillations of twenty-four-hour amplitude of temperature are usually in May-June, and the lowest - in December. The lowest temperature is usually observed in winter during the northern and north-east winds. In the summer the hot weather brings about the east and south-east winds [8].
Atmospheric Precipitations and Snow
The atmospheric conditions are formed by circulation of air mass on the whole. Average annual amount of precipitation with correction for the moistening of the draught gauge is 638 mm. During the warm period of the year (April-October) about 70 % of all precipitation takes place, and during the cold period (November-March) - about 30 %. The coefficient of variation of multi-year annual precipitation is 0.15. Minimum of precipitation occurs in March, and the maximum - in July-August. There are about 170-180 days with precipitation (0.1 m and more) per year [8]. The snow cover in the region is about 100-110 days per year. Average height of snow cover is 30-40 cm [8].
Evaporation and Humidity
Multi-year amount of annual evaporation from the dry land is about 500 mm, evaporation from the water surface during the warm period (April-November) is about 600 mm with the coefficient of variation 0.15 [8].
Average relative humidity of air reached 80 %, and about 90 % in winter. A minimum relative humidity (53-63 %) is observed in June, and a maximum - in January [8].
Fog and Oscillation of Atmospheric Pollutants
In the Ignalina NPP area, fog is observed during the entire year. Average number of foggy days is 45 and a maximum - 62 days. Fog absorbs different impurity (noxious gases, smoke, dust) and, combined with high humidity, increases corrosion intensity, aggravating visibility and impeding transportation. Average duration of fog in the course of a month is from 4 to 29 hours and in the course of year is about 173 hours. During the cold period total duration of fog oscillates between 92 to 106 hours, and during the warm period it is about twice lower which is 49-68 hours.
Maximum dust intensity is observed in May, and minimum-in December. Oscillations of total content of sulphurous gas has the following annual distribution: lower values are observed in the summer and autumn, and highest ones-during the cold period of the year.
Ground Freezing
The ground usually begins to freeze in the first part of December and lasts to the middle of April. Average depth of the frost line reaches about 50 cm, with a maximum extending to 110 cm depending on the composition of the ground and its humidity.