The Atmospheric Emitted Radiance Interferometer (AERI) measures downwelling longwave radiance with approximately 1 wavenumber resolution from 3.3 to greater than 18 microns. Throughout this spectral region, the absorption coefficient for liquid water and ice varies significantly. Furthermore, at some spectral locations the absorption coefficients are approximately equal, but they can be substatially different at other wavelengths. By selecting narrow microwindows between absorption lines, cloud phase can be determined and microphysical properties can be retrieved. Results from the AERI at the ARM Southern Great Plains (SGP) and North Slope of Alaska (NSA) sites will be shown and discussed.