Cloud and aerosol properties were observed by aircraft in autumn over the Beaufort Sea during the 1994 Beaufort and Arctic Storms Experiment (BASE). The microphysical properties (particle size, concentration, mass, and phase) and vertical structure of autumn clouds are examined as a function of height and minimum in-cloud temperature, Tmin. Below 2 km, liquid clouds were observed at Tmin between -5¿ and -9 ¿C, mixed-phase clouds were observed between -5¿ and -20 ¿C, and clear-sky ice crystal precipitation was observed at Tmin as warm as -14 ¿C. Between 2 and 5 km all clouds were mixed-phase and typically consisted of a thin layer of liquid with ice extending well below the liquid layer. These mixed-phase clouds were found at Tmin as low as -32 ¿C. All clouds observed above 5.5 km were composed entirely of ice at Tmin as warm as -33 ¿C. The concentration of ice crystals is observed to increase exponentially with decreasing Tmin. The Hallet-Mossop ice multiplication process did not appear to be an important in the production of ice crystals in the mixed-phase cloud observed in this study. The atmosphere was relatively clean with condensation nuclei (CN) concentrations rarely exceeding 300 cm-3. The smallest CN concentrations (as low as 50 cm-3) were observed in the boundary layer and just above the surface where precipitation and nucleation scavenging have cleansed the air. Thin layers of very large CN concentrations were often observed within and just above low-level clouds possibly resulting from gas-to-particle conversion which requires clean and humid air typical of lower Arctic atmosphere. ¿ 2001 American Geophysical Union