We interpret lidar observations by Browell et al. (1990) using single scattering calculations for nonspherical particles and aerosol microphysical calculations. Many of the lidar observations are consistent with particles containing 10 ppbv of condensed nitric acid vapor and an equivalent mass of water. The lidal observations of these Type 1 clouds identify two subtypes, whose properties we deduce. Type 1b particles are spherical, or nearly spherical, and typically have radii near 0.5 &mgr;m; Type 1a particles are not spherical, and have a spherical volume equivalent radius exceeding 1.0 &mgr;m. Several factors may cause variations in the size of the particles. The most significant factors are the cooling rate and the degree to which the air parcels cool below the condensation point. Specific examples in which cooling rate and cooling point may have led to variations in particle size are found in the Browell et al. (1990) data set. Condensation of 1 ppmm of water or less is quantitatively sufficient to account for the magnitude of the lidar backscatter observed from water ice clouds. The ice particles are not spherical in shape. The sizes of particles in water ice clouds cannot be determined because they are much larger than the wavelength of the lidar. ¿ American Geophysical Union 1990