The shapes of very small cirrus particles derived from in situ measurements

The shapes of ice crystals have been derived from measurements in cirrus during the CRYSTAL/FACE experiment in July, 2002. The measurements were made in the size range from 5–45 μm with an optical particle spectrometer that measures forward (4–12°) and backscattered (168–176°) light from individual particles. The phase functions for ensembles of ideal crystal shapes were integrated over these collection angles to compare with the measured ratios of forward to backward scattered light from nine research flights. Approximately 90% of the crystals agreed with the theoretical model for a mixture of bullet rosettes, plates and hollow columns. Approximately 10% of the crystals were characterized as ice spheres. There was no significant dependency of shape on wind direction, temperature or relative humidity with respect to ice.