Observations of vertically thick polar stratospheric clouds and record lowtemperature in the Arctic vortex

On 12 and 13 January 2001 backscatter sondes launched at Sodankyla, Finland (67 degrees N, 27 degrees E) detected an extraordinarily thick polar strat ospheric cloud layer of more than 8 km vertical extent. On these days the p olar 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 radios onde measured record low temperature of 176.7 K at an altitude of 25.2 km a t the vortex edge. High vertical resolution radiosonde profiles and meteoro logical analyses indicate strong mountain wave activity on this day. This p rovides further evidence that the coldest temperatures in the Arctic lower stratosphere occur as a consequence of mountain wave cooling under cold syn optic-scale background conditions.