Halos Indicate Cubic Airborne Ice

Photographs of atmospheric halos over the Lascar volcano in northern Chile imply that water ice can grow in a cubic crystalline structure. Researchers from the Ursa Astronomical Association of Helsinki, Finland, and Helsinki University of Technology of Espoo, Finland, imaged the rings of light in November 1997. They suggest that temperatures taken at Antofagasta, a town west of the site, indicate that the local conditions were suitable for the growth of cubic ice.

The reflection and refraction of light through atmospheric ice in tropospheric clouds produce the rings and arcs of radii proportional to the crystalline structure of the ice. Reporting in the Nov. 20, 2000, issue of Applied Optics, the team noted that hexagonal ice crystals produced all the other documented halos but that the existence of the naturally occurring cubic form has been proposed for decades.