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Halo displays in the sky contain valuable information about icecrystal shape and orientation: e.g., the 22° halo is producedby randomly oriented hexagonal prisms while parhelia (sundogs)indicate oriented plates.  HaloCam, a novel sun-tracking camera systemfor the automated observation of halo displays is presented.  Aninitial visual evaluation of the frequency of halo displays for theACCEPT (Analysis of the Composition of Clouds with ExtendedPolarization Techniques) field campaign from October to mid-November2014 showed that sundogs were observed more often than22° halos.  Thus, the majority of halo displays was producedby oriented ice crystals.  During the campaign about 27 % ofthe cirrus clouds produced 22° halos, sundogs or upper tangentarcs.  To evaluate the HaloCam observations collected from regularmeasurements in Munich between January 2014 and June 2016, anautomated detection algorithm for 22° halos was developed,which can be extended to other halo types as well.  This algorithmdetected 22° halos about 2 % of the time for thisdataset.  The frequency of cirrus clouds during this time period wasestimated by co-located ceilometer measurements using temperaturethresholds of the cloud base.  About 25 % of the detectedcirrus clouds occurred together with a 22° halo, which impliesthat these clouds contained a certain fraction of smooth, hexagonalice crystals.  HaloCam observations complemented by radiative transfersimulations and measurements of aerosol and cirrus cloud opticalthickness (AOT and COT)provide a possibility to retrieve more detailed information about icecrystal roughness.  This paper demonstrates the feasibility ofa completely automated method to collect and evaluate a long-termdatabase of halo observations and shows the potential to characterizeice crystal properties

Ice crystal characterization in cirrus clouds: a sun-tracking camera system and automated detection algorithm for halo displays