Occurrence of polar stratospheric clouds at Kiruna

Polar stratospheric clouds (PSCs) are often observed in the Kiruna region i n northern Sweden, east of the Scandinavian mountain range, during winterti me. PSC occurrence can be detected by ground-based optical instruments. Mos t of these require clear tropospheric weather. By applying the zenith-sky c olour index technique, which works under most weather conditions, the data availability can be extended. The observations suggest that PSC events, esp ecially of type II (water PSCs) may indeed more common than predicted by sy noptic models, which is expected because of the frequent presence of mounta in-induced leewaves. However, it will be of importance to increase the dens ity of independent observations.