The Institute for Marine and Atmospheric Research Utrecht (IMAU) participated in the FIRE III (First ISCCP Regional Experiment, ISCCP International Satelite and Cloud Climatology Experiment) experiment in May 1998. On the sea ice at the SHEBA camp we performed surface-based measurements of longwave and shortwave components of the radiative flux, UV-B radiative flux and the liquid water content and effective radius of the droplets. A sonic anemometer and Lyman-alpha measured the wind, temperature and moisture with a frequency of 20 Hz. From the data we will calculate the turbulent fluxes of heat, moisture and momentum. Together with measurement from a 9 meter high profile mast we will study the turbulence characteristics in the surface layer (flux-profile relationships, roughness lengths, etc.). A vertical profile of the atmosphere was obtained by instruments attached to a helium-filled balloon which provide temperature, wind velocity and direction, relative humidity and (visible light) actinic flux. The IMAU measurements and aircraft data will be analysed to study the influence of different physical processes on turbulent mixing, radiation and microphysics in a cloudy Arctic boundary layer. Special attention will be given to cases in which the cloud extended down to the surface (fog)