Resumen de Sky smiley face: Solutions for Fermi Questions, November 2024
Professor, look at the rainbow in the sky!” I was walking across campus one beautiful (and apparently clear) November morning. Two days prior, I had been explaining to a class the calculus-based mathematics underlying rainbow formation. A student* in that class greeted me with the above remark that morning, and I followed her outstretched arm until I noticed the multicolored smile near the zenith—probably at the most about 10° of arc from the zenith. It was a circumzenithal arc (CZA) [Fig. 1(a)], formed by specific light ray paths through hexagonal plate ice crystals populating thin cirrus clouds. Crystals giving rise to the CZAs are predominately horizontally oriented as they fall due to the aerodynamic forces around them. They are only visible in favorable conditions (i.e., cirrus clouds being present) when the Sun’s altitude φ is about 32° of arc or less.
