is an ice-based atmospheric effect
. It is the most common subhorizon halo
. It appears as a white spot or column of pure white light at an angle below the horizon
exactly as far as the sun
is above it.
The effect is caused when plate-shaped ice crystals are uniformly distributed below the observer to form a reflective surface, as in freezing fog or across flat-topped clouds as seen from a plane or a mountain. These types of ice crystals are formed in temperatures between 0 and -4° C. The surface is imperfect of course and subsuns tend to appear more circular when the sun is at higher elevations and more teardrop-shaped when the sun is at lower elevations.
Similar conditions can result in subparhelia and sun pillars.