An Introduction to Sun Pillars and Related Phenomena
Ice crystals form in a wide variety of shapes and sizes: stars, needles, columns and plates. When bright light passes from the sun, moon or an artificial light source through a portion of the sky containing a concentration of ice crystals, magical apparitions often appear. The crystals focus, scatter, bend, split and reflect the light rays into a kaleidoscope of optical phenomena: arcs, glories, halos, pillars and sun dogs.
Sun Pillar: Photo Courtesy of NOAA/US Dept of Commerce
One particularly interesting form is the light pillar. They are most commonly seen as sun pillars; however, moonlight and strong artificial light such as street lamps can also form light pillars. The pillar appears as a feather of light that extends vertically above and/or below the light source.Most sun pillars are seen when the sun is low on the horizon (generally no more than 6 degrees above) or just below it.
Sun pillars usually extend only 5 to 10 degrees of arc directly above the solar disk and are rarely seen as high as 20 degrees above the solar disk. Moon pillars form in the same fashion as sun pillars. Pillars form from artificial lights in a similar manner, but the closeness of the source to the observer can allow larger pillars to form.
Sun Pillar: Photo Courtesy of NOAA/US Dept of Commerce
Pillars appear in the sky when snow or ice crystals reflect light forward from a strong source such as the sun or moon. Those crystals with plate or column shapes provide an excellent surface from which the light may reflect toward the viewer's eyes. Ice crystals in the form of plates or columns can be found in ice clouds (cirrus or alto forms), ice fogs, snow virga falling from high-based clouds, blowing snow and diamond dust.
Because the light rays forming pillars are reflected, they take on the colour of the incident light. For example, when the sun is higher in the sky, pillars are white or bright yellow in colour. But when it is near the horizon and its light colour dominantly orange, gold or red, so is the resulting pillar.
Figure 1: Ice Plates: Photo by W.A. Bentley
The formation of hexagonal-plate crystals is favoured at air temperatures from 0° C to -4° C (32° F to 25° F) and from -10° C to -20° C (14° F to -4° F). Ice plates resemble dinner plates with a hexagonal pattern in their long dimension and are thin relative to their width (Figure 1 shows some examples of ice plates). When their size is less than 15-20 micrometres across, crystal plates tumble randomly through the air as they fall. But when the plates are larger, they fall so that their long dimension parallels the ground, floating downward like descending flying saucers.
Figure 2: Ice Columns: Photo by W.A. Bentley
Ice columns resemble stubby pencils rather than the delicate branched snowflake shape our mind conjures up at the first mention of snow. Columns typically form in the temperature ranges -5° C to -8° C (23° F to 18° F) and below -25° C ( -13° F). They are long in comparison to their hexagonal cross-section (Figure 2). Larger column crystals also fall with their long axis paralleling the ground. At times, the columns may rotate while falling like slow, miniature helicopter blades.
When the larger plates and columns assemble in a stable "flying formation" and strong light passes through the formation from the source to the observer, the ice crystals can act like small mirrors. Pillars appear when the light rays reflect at a grazing angle as they hit the crystal's upper or lower surfaces (Figure 3). Light can also be reflected forward as it passes through thin plates.
Figure 3: Reflection of Light from Crystal Surface
The breadth, form and location of a light pillar depends upon the type and orientation of the ice crystals, their height in the sky and distance from the observer, and the altitude of the light source. Plates generally only produce sun pillars visible on the ground when the sun is within 6 degrees of the horizon. Light reflected off ice columns can form pillars visible on the ground when the sun is higher, but rarely when the sun is more than 20 degrees above the horizon.
Sun Pillar: Photo Courtesy of NOAA/US Dept of Commerce
Plates roughly 1 millimetre across align their long axis nearly perfectly in the horizontal plane as they fall and thus form narrow, bright pillars. If crystal alignment deviates slightly from perfect alignment, the resulting pillar broadens and may be detached from the light source.
When the ice crystals such as those in or falling from a cloud are at great distance from the observer, some slight misalignment of the crystals from the horizontal is required for pillars to appear (Figure 4). Light rays reflecting from distant, perfectly horizontal plates would reflect upward or downward away from the observer's line of sight or simply pass through the crystal to the observer's eye, thus not forming a visible pillar. When crystals are tilted slightly, the light can reflect off the lower outer crystal surface (or inner upper surface) toward the observer's eye.
Figure 4: Pillar Formation from Low Sun
When the sun is low in the sky (one or two degrees above the horizon), reflections can occur off the lower surface of perfectly horizontal crystal alignment (Figure 5). Because such pillars are often very narrow, their subtle structure can be lost amid the brilliance of a golden or crimson sunrise or sunset.
Figure 4: Pillar Formation from Setting Sun
For closer sources such as street lighting, plate orientation is not as critical. When they have a wide range of alignments, the pillar formed is much broader, giving the pillars a sheet-like appearance (Figure 6). Such pillars can vanish and reform with amazing quickness if gusty winds rapidly rearrange the crystal alignments, thus giving the impression of a shimmering aurora overhead.
Figure 6: Broad Pillar Formation from Street Light
While sun pillars are rarely more than 20 degrees in height, pillars forming over artificial light sources are not restricted to a visual height angle of 20 degrees. Such pillars may appear as tall as 90 degrees depending on the height of the reflecting crystals and location of the observer.
When the crystals are high in the sky -- in clouds or snow virga -- only reflections off the bottom of the crystals will form pillars visible to an observer at the ground. However, if viewed from a mountain top or aircraft, pillars formed by reflections from crystal upper surfaces can also be seen. From such vantage points, pillars extending below the light source are common.
When the crystals are present near the near the surface such as during light falling snow, diamond dust and blowing snow, reflections may be seen from the upper surface of some crystals as well as the lower surface of others. This can produce pillars which extend both above and below the light source. Street lighting can form such pillars. Figure 7 shows how this happens.
Figure 7: Pillar Formation from Street Light
Sun and moon pillars are not restricted to the cold season since high altitude clouds can contain ice crystals. At times pillars occur simultaneously with other optical phenomena generated by ice crystals such as halos and sun dogs. However, blowing snow, ice fog and diamond dust can not only produce interesting pillars with sun or moon light, but also generate very interesting effects around strong artificial light sources.
Sun Pillar: Photo Courtesy of NOAA/US Dept of Commerce
Sun Pillar: Photo Courtesy of NOAA/US Dept of Commerce
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