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  Plate Pillars - Formation 
  









Most sun pillars are formed by plate-like crystals.

Pillar forming plates range from the ideal crystals at left to large and wobbly semi-snowflake varieties.

Optical perfection is unnecessary. Often pillars are the only halo in the sky because crystal faults like side faces blocked by growths or internal defects prevent other arcs from forming.

Upper pillars are created by rays reflected downwards from the lower faces of tilted crystals. In plate crystals of good quality, a significant number of pillar forming rays enter a side face, undergo total internal reflection an odd number of times from the upper and lower basal faces and then leave through the opposite side face. The refraction at the two faces is equal and so the pillar is not coloured. However, pillars adopt the colours of the incident sunlight which may be highly reddened.

Lower pillars form when light is reflected upwards from the topmost crystal face.

Most halos are at their finest when crystals are well aligned. In contrast, sun pillars need crystals with large tilts. The larger the tilts the taller is the pillar.








Ray paths that create upper and lower sun pillars.