The high-resolution spectra of the Atmospheric Infrared Sounder (AIRS) provide a global view of small- particle-dominated cirrus clouds, and they exist over much larger spatial extents than seen in previous aircraft campaigns. As shown by simulations using a plane-parallel scattering radiative transfer (RT) model and realistic ice particle shapes, the shape of the radiance spectra in the atmospheric windows is uniquely influenced by small ice crystals. Minima in the brightness temperature (BT) spectra between 800 to 850 cm-1 are seen for ice particles smaller than 3 ¿m in the RT simulations and AIRS spectra. A case study of an orographic cirrus cloud observed on October 2, 2002, over the central Andes of South America is presented with spectral BT differences up to 63K between 998 and 811 cm-1.