1000 and 1800 m and high above 1800 m. The low altitude zone is heavily influenced by the Mediterranean climate, the influence being more pronounced on the western slopes facing the Struma valley than on the eastern slopes along the Mesta valley. The high altitude zone is characterised by low temperature, low temperature amplitude, many cloudy days, lasting and thick snow cover, strong winds and intensive sun radiation. Winters are cold and prolonged while summers are cool and short. The temperature decreases with the altitude, which is more visible in summer. The mean annual temperature is around 9–10 °C in the low, 5–7 °C in the middle and 2–3 °C in the high altitude zone. 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 in Pirin is 600–700 mm in the lower zones and 1000–1200 mm in the higher. 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 altitudes 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 are frequent. The prevailing wind direction is from the west and the north-west but southern and south-western winds are also frequent. The wind speed can reach 34–40 m/s at the high ridges, being highest in February and March and lowest in August and September. The percentage of windless days increases in summer and autumn to 30–40% at lower altitude and 10–15% at higher. In the highest zone the percentage does not exceed 5–7% annually. Hydrology Pirin forms the water divide between the basins of the rivers Struma and Mesta. The watershed follows the main ridge of the mountain in direction north-west to south-east. As the main ridge is situated closer to the Mesta valley, the tributaries to the Struma tend to be longer. The