Interrelationships between volume extinction coefficient (σ), ice water content (IWC), and effective radius (re), each dependent upon the particle size distribution (PSD) and temperature (T), are developed using in-situ microphysical measurements at low and mid-latitudes, remote sensing data, and model results. The ratio <$frac{{rm IWC}}{sigma}$>, proportional to re, increases with T. Lower values of <$frac{{rm IWC}}{sigma}$> are observed near cloud top and base than within a cloud layer, although <$frac{{rm IWC}}{sigma}$> changes more with temperature than relative height within the cloud. For equivalent radar reflectivities (Ze) below about -28 dB, the minimum detectable with forthcoming spaceborne cloud radar, <$frac{{rm IWC}}{sigma}$> is a nearly constant value. IWC increases almost linearly with σ, with a temperature-dependence noted.