Optical properties of contrail-induced cirrus: discussion of unusual halo phenomena

Photographs of a 120 [degrees] parhelion and a 22 [degrees] parhelion within persistent contrails are presented. These phenomena result from hexagonal plate-shaped ice crystals oriented horizontally with diameters between 300 [[micro]meter] and 2 mm. From our observations and reinvestigation of previous reports, we conclude that a subset of the population in persistent contrails can consist of highly regular, oriented, hexagonal plates or columns comparable to the most regular crystals in natural cirrus clouds. This is explained by measured ambient humidities below the formation conditions of natural cirrus. The resulting strong azimuthal variability of the scattering phase function impacts the radiative transfer through persistent contrails. Key words: Aircraft, contrail, climatic impact, crystal growth, halo phenomena, ice crystals, optical properties, relative humidity, remote sensing, scattering phase function.