Thin layers of large (r≈2--3 &mgr;m) particles were observed in the Arctic between 14 and 27 km on 23 and 30 January 1989. Although the particle mass is consistent with the available HNO3 vapor, it is not always clear whether the morphology of the layers is the result of recent condensation or evaporation as the layers appear under conditions of both supersaturation and undersaturation with respect to nitric acid trihydrate (NAT). Large amplitude, small vertical scale temperature variations appear to be associated with the layers but not in a simple manner. In regions supersaturated with respect to NAT, condensation on only a few of the available condensation nuclei takes place. Such preferential condensation into thin large particle layers suggests that this class of nuclei may be different than the previously assumed sulfate aerosol nuclei. The possibility that they may be derived from solid fuel rocket effluent and spacecraft ablation products is examined and cannot be excluded. Such a source may explain the appearance of these cloud layers at high altitude. ¿ American Geophysical Union 1990