Iridescence in an aircraft contrail

The available diffraction-corona theory for the interpretation of the cloud iridescence phenomenon is reviewed and applied to photographic observations of an iridescent contrail. It is concluded that the simple-diffraction theory qualitatively explains the occurrence of corona and iridescence under the cloud microphysical conditions with which these phenomena are typically associated, and that the theoretical predictions of cloud droplet diameters of 1–3 μm during initial contrail formation appear to be reasonable for a highly supersaturated environment. However, additional Mie theory simulations utilizing narrow droplet size distributions should be performed to assess the impact of anomalous diffraction in realistic cloud compositions in order that iridescence observations may be more precisely interpreted for cloud microphysical studies.