Light Pillars, a Million-Mirror Optical Illusion on Winter Nights
Don’t worry. It’s totally natural. There’s no mother ship hovering above Earth, waiting to carry you away, and no demon shooting light from beneath, signaling to pull you below. It’s a light pillar—or a few of them—a colorful column of sparkling light that appears to beam up toward the sky. It’s all just an icy illusion. And now, during winter, when nights are long and cold, you have a good chance to spot one as long as conditions are right.
If you’re lucky enough to spot a light pillar, what you’re seeing is actually artificial light from a ground source reflecting off millions of floating ice crystals.
“It’s not an upward beam of light,” said Les Cowley, a physicist who runs a website on atmospheric optics, or the way light travels through the atmosphere. “Although they look pretty, they’re also a sign that someone, somewhere could do better with their lighting. You might call them light pollution pillars if you wanted to be environmental about it.”
Light pillars occur when ice-containing clouds—normally miles high in the atmosphere—cling closer to Earth’s surface, just a hundred or a few thousand feet above it. These thin clouds contain millions of flat, hexagonal crystals of ice that float horizontally in the air. Each ice crystal acts like a mirror pointed downward, reflecting the artificial light back to your eyes—as long as the cloud is about halfway between you and the light source. Together, the crystals form a cluster of mirrors floating at different heights, which allows you to see the light as a column; if they all were at the same level, you’d see only a spot of light. The pillars can be quite colorful at times because each reflected beam is the same color as its source.
To find a good source—one bright, close to the ground, and just far enough away—your best bet is to travel a few miles outside your city or town on a dark night. The weather doesn’t have to be frigid, but it helps. Light pillars are frequently spotted in Scandinavia, upstate New York, Canada and other cold spots in the winter. But Dr. Cowley said that this week he received a photo of light pillars from about 30 miles outside of a town in Iran—perhaps because the desert can get cold at night.
If leaving the city is not an option, look up at streetlights on a freezing cold night. You may spot the sparkly, ghostly blur of diamond dust. This illusion is created by the same flat ice crystals that produce light pillars, but this time, they’re so low to the ground you can feel them prickling your skin. “When that happens, you see other nearby halos, and it’s quite a spectacular sight,” Dr. Cowley said.
 Source: The New York Times (January 17, 2017)