The summertime formation and burial of low-density, coarse-grained, hoar layers in the snow of central Greenland can be mapped using satellite passive-microwave data. Variations in a signal derived from Special Sensor Microwave/Imager (SSM/I) brightness temperature trends correlate temporally and spatially with hoar complex evolution over ≈120 000 km2 of the Greenland ice sheet's dry firn facies. Observations at the Greenland Ice Sheet Project II (GISP2) site indicate that changes in surface conditions and brightness temperatures correspond to four hoar events over two summers, as expected from theory. Following a snowfall, the smooth, high-density surface reflects some emitted, 37 GHz, horizontally-polarized (H) radiation but little vertically-polarized (V) radiation. Progressive surface roughness increases and density decrease during hoar formation cause a gradual decrease in the H reflection. Formation and burial of a hoar layer thus causes a slow decrease followed by an abrupt increase in the V/H ratio. Hoar layers have been used to date the GISP2 ice core through the entire Holocene; archived microwave data now can be used to assess the timing and frequency of the formation of these stratigraphic markers in central Greenland. ¿ American Geophysical Union 1993