Measurements of UV aerosol optical depth in the

Routine measurements of global and diffuse UV irradiances at Briancon station (1310 m a.s.l.) are used to re- trieve the direct solar irradiance and the aerosol optical depth (AOD), for cloudless days. Data of three years (2003, 2004, 2005) are analyzed; the results confirm those of a preliminary analysis for 2001, 2002. The atmosphere is very clear in winter, with AODs be- tween 0.05 and 0.1. The turbidity increases slowly in spring, starting end of February, with AODs around 0.2-0.3 in mid summer, some values reaching 0.4. A similar behaviour is observed for all years, with somewhat higher values in late summer for the year 2003.