The average precipitation in Bulgaria is about 670 mm. It is uneven in terms of seasons and territory. In northern Bulgaria the highest precipitation is in May–June, while in southern Bulgaria it is in winter. The average amount of precipitation also varies in term of altitude – from 450–850 mm in the plains to 850–1200 mm. The lowest mean precipitation is in the eastern part of Dobrudzha and the Burgas Plain (450 mm) and in the area between Plovdiv and Pazardzhik (500 mm); the highest rainfall falls in the mountains – the Petrohan Pass in the western Balkan Mountains and Zlatograd in the Rhodope Mountains. The highest annual rainfall was measured in 1957 in the upper valley of the river Ogosta in the western Balkan Mountains (2293 mm); the highest daily rainfall was recorded at Saints Constantine and Helena resort (342 mm) near Varna in 1951. The total annual amount of the rainfall is 74 billion km3; of them 70% evaporate, 20% flow into the rivers and 10% soak into the soil. Most of the country is affected by droughts in June and August. The snow cover lasts from 20–30 days in the lowlands to 9 months in the highest mountains.