[1] The sea ice fraction in the interior of the pack, especially in winter, is important for climate studies and for validation of sea ice models. When ice concentration is high, a difference of only a few percent in ice fraction can have a major effect on the ocean-air fluxes. Satellite estimates of the ice fraction in the Antarctic winter based on microwave emissions are typically 90–95%, with values from the “Bootstrap” algorithm higher than the “NASA team”. However coupled ocean-atmosphere models usually show higher concentrations. A recent evaluation for the Arctic has shown that the true winter ice fractions can be very high, above 99%. Upward looking sonar data from the Weddell Sea is used to show that Antarctic concentrations are higher than previously estimated, and to reevaluate climate model results in this light. The bootstrap algorithm is found to provide a better fit to the sonar data.