The objective of the ACSYS 1998 field experiment was the investigation of the atmospheric boundary layer in case of on-ice air flow in wintertime. Cyclones, reaching the Arctic sea ice from the Greenland Sea or Barents Sea are the strongest synoptic-scale weather signals in the Arctic region. They transport warm, moist and cloudy air from the open water to the Arctic ice shield. Especially in wintertime when the temperature contrast between the ocean and the ice surface is large the effects in the boundary layer over the ice are also significant. The stable and cloudfree boundary layer of the 'undisturbed' winterly Arctic is modified to an also stable but cloudy boundary layer if a cyclone transports warm air on the ice. To quantify this boundary layer modification was the objective of the field campaign ACSYS 1998.