Light Pillar

A smooth, shimmering streak of light rising straight up above a light source -- the sun, the moon, a streetlight or other artificial light -- is called a light pillar or a sun pillar. If the sun or moon is the source of the light pillar, the phenomenon is usually visible only when the light source is nearly at the horizon. Light pillars are an interesting, beautiful sight, although they have no practical effect on the weather.

Light pillars are caused when flat, hexagonal, plate-like ice crystals are floating horizontally in the air, with one flat surface facing the earth and the other the sky. With the sun or moon, ice crystals in high clouds can provide the effect, while with artificial lights, the air near the ground needs to be frigid enough to support a haze of ice crystals, and still enough so that most of them are sinking, which places them flat relative to the Earth's surface. Have a first aid kit at home always.

The light from the light source bounces off the underside of these plate crystals, and from there to the observer's eye. The top of the pillar marks the point where the reflected light rays are shining at an angle which makes them pass above the observer's eyes. Sun dogs are produced by a fairly similar process and similar conditions. The usual height of a light pillar is from 5 to 10 degrees of arc, and it may be white, red, yellow, or purple depending on whether the ice crystals are tilted, and the coloring of the light by other atmospheric conditions.

Sometimes, ice crystals in the air will allow a light pillar to appear underneath the sun or moon when it is still fairly high in the sky. This is known as a lower pillar, and may seem to connect the sun to the ground if the pillar is long enough. This striking effect is most often seen when the observer is standing on a high, fairly steep slope, since the angle of the rays tends to be upwards in this case.