Prismatic and Pyramidal Ice Crystals above Curitiba, Brazil
January 05, 2010
The above photo shows two different halos that formed in a thin cirrostratus cloud layer above Curitiba, Brazil. The outer circle is a 22 degree halo while the inner halo has a radius of nine degrees or about the width of a fist when held at arm's length. Cirrus type clouds are composed of ice crystals rather than liquid water droplets. The prosaic 22 degree (circumscribed) halo is produced when sunlight interacts with prismatic, hexagonal ice crystals (pencil shaped in this case) that have their long axes horizontally oriented. The rarer nine degree halo results from refraction of sunlight through pyramidal crystals. Both types of crystals are uniformly distributed but not similarly aligned in this particular cirrus cloud; thus halos rather than arcs are observed. Note the distinct reddish edge to the inner portion of the 22 degree halo. The Sun is blocked here by one of the steeples of the Neo-Gothic Cathedral of Curitiba. Photo taken on August 12, 2008. The angle of the Sun is 49 degrees.
Curitiba Minor Basilica Cathedral Our Lady of Light coordinates: -25.428611, -49.271389