**4. Meteorological condition**

The climate of the karst region as everywhere in Lithuania is intermediate between the mari‐ time and continental climates. The annual infiltration of precipitation exceeds evaporation [8].

Air temperature observations in the active karst zone were started only in 1924 [12].

According to the data of Biržai Meteorological Station, the long‐term annual averages of the main climatic characteristics are as follows: air temperature 6.6°C; precipitation 661 mm. The mean temperature for July, the warmest month of the year, is 18°C, and for January, the cold‐ est month, it is 3.6°C below zero. The rainfall during a warm season, from April to October, makes up 80% of precipitation. Long‐term mean monthly amount of precipitation is varying from 36 to 81 mm.

Snow appears at the end of November and melts at the end of March. Thickness of snow cover reaches 6–44 cm. The mean depth of freezing is varying 17–99 cm.

During the last decades of the twentieth century, winter season temperature particularly near ground surface has increased significantly [12]. A distinct increase in air temperature has been observed since 1960. In karst region, the average temperature of January and February increased in the period 1961–1990 compared with the period 1931–1960 by 0.5°C [13].

The researchers [12] state that mean temperature in summer decreased until 1980 and then started to increase. Significant increases in annual, winter and summer temperatures were observed in 1994–2004. Increases in air temperature, during both cold and warm seasons, influenced other hydrometeorological characteristics and the solubility of gypsum.
