A subsun 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.

  • http://amsglossary.allenpress.com/glossary/search?p=1&query=subsun
  • http://www.meteoros.de/arten/ee44e.htm
  • http://www.sundog.clara.co.uk/halo/subsun.htm

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