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The surface radiance spectrum within the terrestrial infrared window (i.e., wavelengths between 8-12 μm or wavenumbers between 833-1250 cm-1) is sensitive to the optical and microphysical properties of cirrus clouds. Numerous microwindows where atmospheric absorption is minimum exist in the spectral regions of 820-960 cm-1 and 1100-1240 cm-1. The minimum radiances at the microwindows in these two spectral regions can be fitted by using two linear lines. The slope of the fitting line for the spectral region of 820-960 cm-1 is sensitive to the effective size of ice crystals within cirrus clouds, whereas the intercept of the fitting line for the spectral region of 1100-1240 cm-1 is sensitive to the optical thickness of the clouds. Based on this spectral feature, a new retrieval method has been developed for simultaneously retrieving cirrus optical thickness and the effective particle size of ice crystals. Furthermore, the ice water path of cirrus clouds can be estimated from the retrieved values of cloud optical thickness and effective particle size.