Two models describing the single-scattering properties of cirrus cloud are tested for physical consistency at solar and thermal wavelengths using airborne high-resolution radiance data. The data were obtained from a case of semi-transparent cirrus cloud, which occurred north of Scotland during October 2000. The single-scattering models tested are randomly oriented hexagonal ice columns and randomly oriented ice aggregates. High-spectral resolution radiances were measured from above the cirrus at a number of wavelengths between 0.3 and 16.7 µm, thereby covering a large range of ice crystal size parameter space and complex refractive index. It is shown that consistency between retrieved optical thickness and ice crystal effective radius at both solar and infrared wavelengths could only be achieved if the ice aggregate model was assumed. Moreover, differences between the ice aggregate model and spectrally resolved brightness temperature measurements were generally well within ±1 K between the wavelengths of 3.3 and 16.0 µm in the clean atmospheric window regions. The paper shows that it is important to have simultaneous radiance measurements from both the solar and thermal spectral regions so that ice crystal scattering models and cirrus retrievals can be rigorously tested. © Crown copyright 2004.