The first direct airborne measurements of the asymmetry parameter (g) and extinction coefficient (βext) of clouds are analyzed. The measurements were obtained with a Cloud-Integrating Nephelometer (CIN) in arctic clouds in May and June 1998. The CIN was evaluated by comparing its measurements of the single-scattering properties of water clouds with values from Mie theory using measurements of the droplet size spectra. These results are used to interpret CIN measurements of the single-scattering properties of ice and mixed-phase clouds. For cirrus clouds composed solely of ice crystals the derived value of g was 0.74¿0.03 (or 0.76¿0.03 if ice crystal faces are assumed to be perfectly smooth and parallel); this value is significantly lower than that calculated assuming the ice crystals to be hexagonal prisms or bullet rosettes. The CIN measurements of βext for cirrus clouds were several times greater than values derived from measurements of the cross-sectional areas and concentrations of the ice crystals. Several possible explanations for the differences between calculated and measured optical properties of the ice clouds are discussed. The measured values of g for mixed-phase clouds depended on the relative concentrations of water and ice. These results are used to show that the albedo of clouds is particularly sensitive to the onset of ice formation. Weak convective clouds and clouds saturated with respect to liquid water generally had higher values of g than those for nonconvective clouds or clouds saturated with respect to ice but subsaturated with respect to liquid water. ¿ 2001 American Geophysical Union