Borovets (1,350 m) and 1200 mm at Musala (2,925 m); about 80% of it being snow at the later. The rainfall occurs mostly in summer and spring on the northern slopes, with maximum in June and minimum in February; the rainfall increases in winter and decreases in summer on the southern slopes and yet the monthly maximum and minimum are the same; there are 130–160 days with rainfall/snowfall. Air humidity in the highlands of Rila ranges between 80 and 85%. The coldest winter months are also the driest. Humidity differs on the northern and southern slopes of Rila. The snow cover above 1,000 m begins to form on 10–15 December on the northern slopes and after 20–30 December on the southern. Its average monthly thickness reaches 20 to 30 cm in February at low elevations. In the Alpine zone above 2,000 m the snow cover is thickest in March, reaching 70 to 80 cm. In the highest ridges the maximum thickness reaches 200 to 240 cm. The snow cover remains for an average of 70–80 days at elevations of 1,200–1,300 m and 180 to 200 days at elevations above 2,000 m. Avalanches are frequent, often caused by temperature changes by the influx of warm Mediterranean air masses. Winds may reach speed of 40–45 m/s (over 100 km/h) at the summits, with mostly south-western and western orientation. The north-west and north-east winds are more moderate. The average monthly wind speed on the highest mountain summits reaches 11–12 m/s. In the lower parts, the average monthly speed varies from 1.2 to 2.5 m/s and in the middle height zone it ranges from 2.5 to 3.2 m/s. Winds are usually strongest in winter and lightest in autumn. The annual duration of sunshine in the Alpine zone is 1930 hours with maximum in August and minimum in December–January; it raises to more than 2150 hours at 1,000 m. Hydrology Rila is an important hydrological unit in Bulgaria with very high hydropower potential, representing about 1/4 of the total potential of the country. The water reserves that form within the mountain range are the most important source of clean potable water for the surrounding settlements, the national capital Sofia and part of the population of Northern Greece and European Turkey. The regime of the rivers of Rila is directly related to the elevation and about half of the water reserves in the mountain are at