A rare optical phenomenon, consisting of a white circle or arc, centred on the antisolar point, and with a reported radius of approximately 33.5°. It is not known whether this is a true halo effect (i.e. through refraction or reflection by ice crystals) or is a misidentified fogbow. Jean Bouguer and Juan de Ulloa were both members of La Condamine's expedition to Peru in 1735, during which they observed the effect.
Subjects: Meteorology and Climatology.