On Feb 5 a very strange case of elliptical halo was recorded by FU Lei from Suzhou, Anhui Province, China.
The short-lived elliptical halo in the display was sliced by the cloud bands into two segments, both exhibit a gradual increase in radius as the halo approaches the cloud bands' lower limits. Such behavior shall be explained by varying apex angles of responsible flat pyramidal crystals.
According to halo experts Marko Riikonen and Nicolas Lefaudeux, it's been long debated whether there's a 'preferred' apex angle in the elliptical halo theory. This particular case serves as strong evidence that there's probably no 'preferred' angle at all. Ellipses can sometimes behave like iridescent clouds - instead of a smooth water droplet size gradient, there is a gradient of crystal angle.
Jia Hao
I have often thought that if the Miller index of ice crystals is very sensitive to changes in temperature and humidity conditions, it might explain such phenomena. It's a pity that I don't know much about crystallography.
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