(16% of the total area), middle between 1000 and 1800 m (40%) and high above 1800 m (44%). The temperature decreases with the elevation, which is more visible in summer. The mean annual temperature is around 9–10 °C in the lower, 5–7 °C in the middle and 2–3 °C in the higher elevation. The coldest month is January with average temperature varying between −5 and −2 °C. The hottest month is July with temperature averaging 20 °C at 1600 m and 15 °C at 2000 m. Temperature inversions, i.e. increase in temperature with height, are observed in 75% of the winter days. The annual precipitation is 600–700 mm in the lower elevation zones and 1000–1200 mm in the higher ones. The rainfall occurs mostly in winter and spring, while summer is driest season. The air humidity is 60–75% in August and 80–85 % in December. In winter the precipitation is mainly snow, varying from 70–90% at the lower elevations to 100% at higher. The average number of days with snow cover varies from 20–30 to 120–160. The highest thickness of the snow cover reaches 40–60 cm at 1000–1800 m in February and 160–180 cm above 1800 m in March (190 cm on Vihren). In some winters the snow thickness can reach 250–350 cm. Avalanches occur frequently in winter. Hydrology The territory of Pirin National Park is almost equally divided between the basins of the rivers Struma (206.06 km2 or 51.1%) and Mesta (197.50 km2 or 48.9%). The watershed follows the main ridge of the mountain in direction north-west to south-east. Pirin is the source of 10 tributaries to the Struma, the largest one being Sandanska Bistritsa, and another 10 to the Mesta. The rivers are short, steep and with high water volume. They form numerous waterfalls which are generally not as high as those in Rila or the Balkan Mountains. The highest one is Popinolashki waterfall, measuring some 12 m. The average annual discharge of the park's rivers is 355,6 million m3, of them 188,5 million m3 flow to the Struma and 167,1 million m3 to the Mesta. The discharge from the park per square kilometre is 2.3 times larger than Pirin's average and 5.6 times larger than Bulgaria's average. The landscape is dotted with 118 permanent glacial lakes, conventionally divided into 17 groups, such as Popovo Lakes, Kremenski Lakes, Banderishki Lakes, Vasilashki Lakes, Valyavishki Lakes, Chairski Lakes, Vlahini Lakes