Optical attenuation measurements in low visibility conditions

Low visibility events detected in the city of Milan during six consecutive winter months are analyzed in order to compare optical attenuation measurements by two different instruments, i.e. a 319 m laser link operating at 785 nm and a visibility sensor. We show that the visibility sensor is a valuable support to estimate link attenuation in the cases when precipitation is absent or there is rain, provided that 1-sample/s raw data are averaged in time. On the other side, in the case of snowfall (that can produce high attenuation values), the agreement between the two sensors is poor.