Light pillars are vertical columns of light that appear to stand above a light source on very cold nights, most often above the street lights, floodlights and industrial lamps of towns in cold climates. They are produced by reflection, not refraction: millions of flat hexagonal ice crystals drifting slowly downward through still, freezing air lie nearly horizontal as they fall, and their upper and lower faces act as a swarm of tiny mirrors. Each crystal reflects a small amount of the lamp's light toward the observer, and because the crystals occupy a range of heights, the reflections merge into what looks like a continuous shaft. Nothing is actually shining upward, and the column is not a beam. It is a virtual image — the reflection of the lamp seen in a vast collection of mirrors, stretched out along the line of sight in the same way that the Moon's reflection in a rippled lake stretches into a glittering path.
Reflection, Not Refraction
This is the point that separates light pillars from the rest of the ice-crystal family. A 22° halo, a sundog and a circumzenithal arc are all refraction effects: light passes through the crystal, is bent by a fixed angle determined by the crystal's geometry, and is dispersed into colours in the process. That fixed angle is why those phenomena appear at fixed, predictable distances from the Sun. The physics is covered in the companion piece on sundogs and the 22-degree halo.
Light pillars involve no passage through the crystal at all. Light simply bounces off the flat basal face, as it would off any mirror, and reflection is not wavelength-dependent. Two consequences follow. First, a light pillar has no fixed angular size — it can be short or extend most of the way up the sky, depending on how deep the crystal layer is and how far away the source is. Second, a light pillar takes its colour entirely from its source. The eerie multicoloured columns photographed above Canadian and Russian towns are simply an inventory of the local street lighting: sodium lamps give orange pillars, mercury vapour gives blue-green, modern LEDs give white, and a stadium or a car park produces whatever colour it emits.
The lake-reflection analogy is exact and worth holding onto. A still lake reflects a lamp as a single point. A rippled lake reflects it as a long vertical streak, because each tilted patch of water surface bounces light to the eye from a different position. A layer of falling ice crystals does the same thing with a horizontal mirror surface suspended in mid-air at many altitudes at once, and the streak stands up rather than lying down because the mirrors are above the lamp instead of below it.
Sun Pillars and Their Artificial Cousins
The natural version came first and is more widely distributed. A sun pillar is a column of light rising above or below the Sun when it is close to the horizon, formed by exactly the same reflection off horizontally-oriented plate crystals in cirrus or in low-level ice fog. Sun pillars are visible far outside the polar regions, are commonest around sunrise and sunset, and often go unrecognised because they are easily mistaken for a bright patch of cloud. Moon pillars occur under a bright Moon by the same mechanism.
Artificial light pillars — the dramatic ones — differ in an important geometric respect. The Sun is effectively at infinity, so its pillar is a genuine sky phenomenon visible identically from anywhere in a region. A street lamp is a few hundred metres away, which means the crystals doing the reflecting are only slightly above the lamp and quite close to the observer. The pillar is therefore local: it exists only for a particular observer at a particular position, it moves as the observer moves, and two people a hundred metres apart see pillars over different lamps. This is why photographs of a townscape crowded with dozens of coloured columns can look composited when they are not — every lamp in view generates its own pillar for whoever is looking.
One asymmetry is worth noting. A pillar can extend below its source as well as above it, since crystals between the observer and a lamp reflect light downward just as readily. Ground observers rarely see the lower half, because the earth is in the way. From an aircraft, or from a ridge looking down onto a valley filled with ice fog, the full double column becomes visible — a lamp or a low Sun with a shaft of light standing both above and below it, symmetrical about the source.
What the Air Has to Be Doing
Light pillars need a specific and fairly demanding set of conditions, which is why they are a signature of extreme cold rather than merely of winter.
- Temperatures around −20 °C or below. Plate crystals of the right habit form readily at these temperatures; warmer conditions tend to produce needles and irregular forms that do not lie flat.
- Very still air. Turbulence tumbles the crystals out of horizontal alignment and destroys the effect immediately. This is why pillars appear on calm, clear, high-pressure nights and vanish in any wind.
- Ice crystals at low level. Usually diamond dust — crystals precipitating directly out of clear, humid, extremely cold air near the ground, rather than falling from cloud.
- A strong temperature inversion, which is what allows the crystal layer to sit undisturbed and which typically accompanies the calm high-pressure conditions required.
The presence of a strong pillar display is therefore a reasonably good visual thermometer. If the columns are tall and numerous, it is very cold and very still, and the same inversion that is holding the crystals in place is likely also trapping whatever the town is emitting near ground level.
Pillar height carries information too. Crystals never lie perfectly flat; they wobble by a degree or two about the horizontal as they settle, and the size of that wobble sets how far up the column extends. A population of crystals tilting by up to about one degree produces a short stub above the lamp; a population wobbling by three or four degrees spreads the reflections over a much greater range of angles and produces the tall columns that reach a third of the way up the sky. Larger crystals settle more steadily and wobble less, so paradoxically it is the smaller crystals, tumbling slightly more, that give the most spectacular pillars.
The Cold-City Belt
Light pillars require both extreme cold and artificial lighting, which restricts them to a narrow set of inhabited places. The interior of Alaska — Fairbanks in particular — produces them regularly, as do Yellowknife, Whitehorse and the Canadian Prairie cities during cold snaps. Northern Scandinavia, Finland, the Baltic states, Belarus, Russia from Moscow eastward, Mongolia, northern Japan and northern China all deliver them. In the United States, Wyoming, Montana, the Dakotas, Minnesota and the upper Great Lakes are the reliable ground.
The requirement for artificial light also gives the phenomenon a curious history: it is far more visible now than it was two centuries ago, because there is far more ground lighting. Sun pillars are ancient; the crowded multicoloured columns above a snowbound town are a modern artefact, and their colour palette has shifted noticeably over recent decades as sodium street lighting has been replaced by LEDs.
Photographing them rewards restraint. The pillars are bright but the scene around them is dark and snow-covered, and the usual failure is an exposure long enough to blow the columns into featureless white stripes. Two to five seconds at a moderate ISO on a tripod is usually plenty, and it is worth deliberately including the source lamps in the frame — a picture of columns alone reads as abstract, while a picture showing the street lights beneath them explains itself. A wide lens helps, because a full display typically spans far more of the sky than expected. The cold is the real technical problem: batteries fail quickly at the temperatures required, and bringing a camera straight back indoors will condense moisture through the lens.
Why They Get Reported as UFOs
Light pillars generate a steady trickle of unexplained-lights reports, and it is easy to see why. They appear abruptly when the temperature drops through the threshold, they are silent and stationary, they hang above the horizon with no visible connection to the ground when the source lamp is hidden behind a building or a ridge, and they come in unnatural colours. Several widely circulated photographs of apparent beams over Russian and Canadian cities have turned out to be pillars over gas flares or industrial floodlights.
The tell is straightforward: a light pillar is always exactly vertical, always directly above its source, and always ends abruptly at the top of the ice-crystal layer. If the column tilts, moves independently, or has structure along its length, it is something else. And if there are several of them, spaced along a road, all the same height and all the same colour, the explanation is standing in plain sight underneath them.
Anyone in a position to see light pillars regularly is in the right place for a good deal else besides — the same cold, calm, clear, high-latitude nights that produce them are the ones that deliver the strongest auroral displays and the most complete diamond-dust halo displays.