Abstract Spectral differences in the extinction between the 10.8 and 12.6 μm bands of the infrared window region, due to optically thin clouds, are observed in the measurements made by a broad-band infrared aircraft radiometer. Similar spectral properties are also revealed by the measurements made by the high-resolution infrared inter-ferometer spectrometer (IRIS) aboard the Nimbus-4 satellite, which had a field of view of ∼ 95 km. These observations show that the extinction due to cloud particles at 12.6 μm is appreciably larger than that at 10.8 μm. Both water or ice particles in the clouds can account for such spectral difference in extinction provided that the particles are smaller than the wavelength of radiation. This spectral effect is demonstrated with the help of multiple scattering radiative transfer calculations. As the IRIS data reveal this spectral feature, about 100 to 200 km away from the center of high altitude cold clouds (∼ 230 K), it is inferred that this feature is related to the sprea...