Q&A: Sun Pillars

Question: Last night at sunset I saw this weird reddish vertical streak of light extending upwards from the Sun. It almost looked like a searchlight beacon. What the heck was that? — SH, New River, AZ

Answer: What you probably saw was a sun pillar. They’re relatively common this time of year, when ice crystals often form in the upper atmosphere. The shifting jet stream is providing those colder upper-air temperatures.

Under certain conditions of humidity, pressure, and temperature, water vapor in the air will freeze into flat hexagonal crystals. These crystals tend to float with their flat sides down due to aerodynamic effects. Irregular hailstones, by comparison, tend to float with more random orientations. The cumulative effect of these floating crystals is similar to what you sometimes see through horizontal window blinds or screens, when an outside light source appears to be smeared in a vertical direction.

The photo above was taken in November of 1999, at sunset, on a particularly brisk day. This sun pillar extended fully 1/3 the way from horizon to zenith (the point directly overhead), was colored deep orange, and had a tight bright shape.

Astronomy is mainly about things beyond the Earth. But sometimes, what happens in our atmosphere can be just as interesting. The interplay of light and air can create some truly beautiful effects. No need to wait for dark to see the splendors of the sky. Many interesting effects are also visible during daylight hours.

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