Abstract The authors derive relations of the equivalent radar reflectivity Ze and extinction coefficient α of ice clouds and confirm the theory by in situ aircraft observations during the First International Satellite Cloud Climatology Project Regional Experiment. Equivalent radar reflectivity Ze is a function of ice water content W and a moment of the size distribution such as the median volume diameter D0. Stratification of the data by D0 provides a set of W − Ze relations from which one may deduce the dependence of particle density on size. This relation is close to that of Brown and Francis and provides confidence in the methodology of estimating particle size and mass. The authors find that there is no universal W − Ze relation, due both to large scatter and systematic shifts in particle size from day to day and cloud to cloud. These variations manifest the normal changes in ice crystal growth. The result is that, with the exception of temperatures less than −40°C, temperature cannot be used to relia...