Ku-band (13.3 GHz) scatterometer and K-band (19.4 GHz) radiometer data acquired by the CCRS CV-580 aircraft over the period from 1979 to 1982 in Canadian and Danish (Greenland) coastal waters have been analyzed to determine the seasonal and regional variations of microwave sea-ice signatures. A clustering analysis of the like and cross-polarized scattering cross sections, Â¿HHo and Â¿HVo, and the H polarized emissivity Â¿H, has been used to identify distinct microwave sea-ice signatures for each ice type and to trace the evolution of these signatures with region and season. Ice-type signatures in the high Arctic under cold conditions are quite stable, and major ice classes are readily identified from microwave measurements. Under warmer conditions the signatures change with the structure, moisture content of the snow pack, and with the free water in the surface layers of the underlying ice. An attempt is made to create a consistent picture of the microwave signature transformation by grouping the data into " seaice seasons" (snow and ice surface transformation stages). The separation between microwave ice-class signatures reaches a minimum at the peak of the summer melt.