This study is based on aircraft data collected on research flights in the Beaufort Arctic Storms Experiment (BASE). The experiment was conducted during the fall, when the frozen parts of the sea surface and the lower troposphere are subject to strong radiative cooling, while the temperature of the open water stays near -1 or -2 ¿C, leading to the formation of two different types of boundary layers. Data from a boundary layer over open water and a boundary layer over frozen sea surface with open leads are examined in detail. The boundary layer over open water was capped by a stratus layer in which large temperature fluxes were observed. Recent advection of air from the colder coastal regions over warm water was probably responsible for the large temperature differences at the sea--air interface. The boundary layer over ice was clear and very stably stratified, with a low--level potential temperature lapse rate in the 20--57 K km-1 range. Turbulent fluxes were very weak or nonexistent, except at the lowest observation level, 40 m above surface, in an area over many open leads. In this boundary layer, wave-like features were common. Some were identified as gravity waves, but others with shorter wavelengths could have been due to horizontal roll vortices generated by convection from the leads. ¿ 1997 American Geophysical Union