There is no great tendency to summer drought, as the flow of air from the north-west brings rain throughout the year. Rainfall is moderate on the foothills and in some sheltered valleys, measuring 700 to 1,000 mm per year, but increases significantly in the higher valleys with levels between 1,000 mm and 1,800 mm. The slopes exposed to the north-west, such as Aulus and Orlu, are, as one would expect, the wettest, together with the frontal ridges that meet air flow from the southwest (giving rise to the Foehn effect). Snow cover is common over 1,000 metres, lasting several months above 1,500 to 2,000 metres. Some periglacial areas exist over 2,500 m but the only true glacier in Ariège is that of Mont Valier, near Castillon-en-Couserans.