Palermo experiences a hot-summer Mediterranean climate (Köppen climate classification: Csa). Winters are cool and wet, while summers are hot and dry. Temperatures in autumn and spring are usually mild. Palermo is one of the warmest cities in Europe (mainly due to its warm nights), with an average annual air temperature of 18.5 °C (65.3 °F). It receives approximately 2,530 hours of sunshine per year. Snow is usually a rare occurrence, but it does occur occasionally if there is a cold front, as the Apennines are too distant to protect the island from cold winds blowing from the Balkans, and the mountains surrounding the city facilite the formation of snow accumulation in Palermo, especially at night. Between the 1940s and the 2000s there have been eleven times when considerable snowfall has occurred: In 1949, in 1956, when the minimum temperature went down to 0 °C (32 °F) and the city was blanketed by several centimeters of snow. Snow also occurred in 1999, 2009 and 2015. The average annual temperature of the sea is above 19 °C (66 °F); from 14 °C (57 °F) in February to 26 °C (79 °F) in August. In the period from May to November, the average sea temperature exceeds 18 °C (64 °F) and in the period from June to October, the average sea temperature exceeds 21 °C (70 °F).