On 12 and 13 January 2001 backscatter sondes launched at Sodankyl¿, Finland (67 ¿N, 27 ¿E) detected an extraordinarily thick polar stratospheric cloud layer of more than 8 km vertical extent. On these days the polar vortex passed over northern Scandinavia. This provided synoptic-scale low stratospheric temperatures leading to the formation of both liquid and solid phase particles. Two days later, on 15 January 2001, a regular radiosonde measured record low temperature of 176.7 K at an altitude of 25.2 km at the vortex edge. High vertical resolution radiosonde profiles and meteorological analyses indicate strong mountain wave activity on this day. This provides further evidence that the coldest temperatures in the Arctic lower stratosphere occur as a consequence of mountain wave cooling under cold synoptic-scale background conditions. ¿ 2001 American Geophysical Union