Antisolar Diffuse Arcs
September 19, 2007
On occasion, rarely observed halos and arcs can be seen when an aircraft over flies high clouds containing ice crystals. The above picture was taken when the Sun elevation was 27 degrees, during a morning flight from Sao Paulo, Brazil, on January 28, 2007. The relatively dark horizontal straight line is simply the aircraft’s contrail shadow projected on the cloud cover. It merges with a rather bright white spot at the antisolar point (point in the sky opposite of the Sun). This spot results from sunlight undergoing a complex set of reflections and refractions through a multitude of ice crystals. With some effort, two diagonal curved lines can be seen crossing at the bright spot. They're known as antisolar diffuse arcs and are also formed from ice crystals.