During April 1972, microwave and infrared data were obtained from the NASA CV-990 research aircraft over the Beaufort Sea ice from the shoreline of Harrison Bay northward to a latitude of almost 81¿N. The data acquired aboard the aircraft were compared with microwave data obtained on the surface at the test site of the Arctic Ice Dynamic Joint Experiment at an approximate position of 75¿N, 150¿W. Over this north-south transect of the polar ice canopy it was discovered that the sea ice could be divided into five distinct zones. The shorefast sea ice was found to consist uniformly of first-year sea ice. The second zone was found to be a mixture of first-year sea ice, medium size multiyear floes, and many recently refrozen leads, polynyas, and open water; considerable shearing activity was evident in this zone. The third zone, in which aircraft and surface data were compared, was a mixture of first-year and multi-year sea ice which had a uniform microwave and signature because the multiyear ice floes were smaller than the instantaneous field of view of the airborne microwave radiometer. The fourth zone was found to be a mixture of first-year sea ice and medium-to-large size multiyear floes which was similar in composition to the second zone. The fifth zone was almost exclusively multiyear ice extending to the North Pole, Zones 3, 4, and 5 are also clearly distinguishable in microwave images obtained from the Nimbus 5 satellite.