A sun pillar is a halo phenomenon of a vertical shaft of light extending upward or downward from the sun. Typically seen during sunrise or sunset, sun pillars form when sunlight reflects off the surfaces of ice crystals associated with thin, high-level clouds (e.g.cirrostratus clouds). The hexagonal plate-like ice crystals fall with a horizontal orientation, gently rocking from side to side as they fall. When the sun is low on the horizon, an area of brightness appears in the sky above (or below) the sun as sunlight is reflected off the surfaces of these tipped ice crystals. However, often only the upper pillar appears.

Sun pillars are seen best within a few minutes of sunrise or sunset. Initially the have about the same coulour and width as the sun, but sunpillars will gradually change their colour from orange-white to red-orange. Sun pillars generally fade 20 to 60 minutes after sunset. Under favourable conditions, e.g. icy fog, sun pillars might stretch out up to 30° above the sun, while light streaks of 5° to 10° are most common.

On rare occasions, regarded with awe in more superstitious times, it may occur with part of the parhelic cycle to give the appearance of a cross centred on the Sun. The sun pillar is comparable with the light course, the setting sun produces on a water surface.