The Beaufort and Arctic Seas Experiment (BASE) occurred during September and October 1994, off the Canadian Beaufort coast. In this paper, we provide a description of the characteristics of the cloudy boundary layer that were observed from a research aircraft during BASE and provide an interpretation for the coevolution of the atmospheric boundary layer and sea ice characteristics during the autumnal freezing of the Beaufort Sea. Observations from a total of 12 research flights are presented. During the course of the experiment the underlying surface made a transition from late summer melt conditions to fullfledged winter conditions. All of the observed boundary layer cloud decks are associated with stable temperature profiles, with temperature inversions occurring either above or within each of the cloud decks. Unlike observations of summertime boundary layer clouds, no multiple-layered clouds in the boundary layer were observed. Examination of the cloud phase indicates a transition from predominantly liquid phase clouds at the beginning of the experiment to predominantly crystalline clouds at the end of the experiment. Local interactions between the clouds and underlying surface were examined in several case studies, including the effects of leads. Using a time series of soundings over a period of 5 days in conjunction with the National Center for Environmental Prediction analyses, an air mass modification event was examined that began after the intrusion of warm air from the Pacific Ocean. The evolution of the air mass is apparent in decreasing temperatures in the lower atmosphere and formation of a low-level mixed phase cloud that evolves with time into a deeper cloud layer that is crystalline.¿ 1997 American Geophysical Union