|Upper & Lower Pillars ~ Ice crystals glint fiercely in the sky of Stokkavatnet by Stavanger, Norway. Imaged Wednesday 20th January '10 by Amelia Heath. The air temperature was minus 6° Celsius and the foreground lake frozen. ©Amelia Heath, shown with permission.
|Most sun pillars are formed by plate-like crystals.
The plates can be 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.