A strange cloud in the Arctic summer stratosphere 1998 above Esrange (68 degrees N), Sweden

When the University of Bonn lidar on the Esrange (68 degrees N, 21 degrees E), Sweden, was switched on in the evening of July 18, 1998, a geometricall y and optically thin cloud layer was present near 14 km altitude or 400 K p otential temperature, where it persisted for two hours. The tropopause alti tude was 4 km below the cloud altitude. The cloud particles depolarized the lidar returns, thus must they have been aspherical and hence solid. Atmosp heric temperatures near 230 K were approximately 40 K too high to support i ce particles at stratospheric water vapour pressures of a few ppmv. The ise ntropic back trajectory on 400 K showed the air parcels to have stayed clea r of active major rocket launch sites. The air parcels at 400 K had travell ed from the Aleutians across Canada and the Atlantic Ocean arriving above c entral Europe and then turned northward to pass over above the lidar statio n. Parcels at levels at +/-25 K from 400 K had come from the pole and joine d the 400 K trajectory path above eastern Canada. Apparently the cloud exis ted in a filament of air with an origin different from those filaments both above and below. Possibly the 400 K level air parcels had carried soot par ticles from forest wild fires in northern Canada or volcanic ash from the e ruption of the Korovin Volcano in the Aleutian Islands.