A novel technique is presented for the detection and mapping of ice polar stratospheric clouds (PSCs), using brightness temperatures from the Atmospheric Infrared Sounder (AIRS) “moisture” channel near 6.79 μm. It is based on observed-minus-forecast residuals (O-Fs) computed when using AIRS radiances in the Goddard Earth Observing System version 5 (GEOS-5) data assimilation system. Brightness temperatures are computed from six-hour GEOS-5 forecasts using a radiation transfer module under clear-sky conditions, meaning they will be too high when ice PSCs are present. We study whether the O-Fs contain quantitative information about PSCs by comparison with sparse data from the Polar Ozone and Aerosol Measurement (POAM) III solar occultation instrument. AIRS O-Fs lower than −2 K generally coincide with PSCs observed by POAM III. Synoptic maps of AIRS O-Fs lower than −2 K are constructed as a proxy for ice PSCs. These are used to investigate spatio-temporal variations of Antarctic PSCs in the year 2004.