Cindy Bidois hit the atmospheric jackpot last week when she ventured to Val Thorens, a ski resort nestled high in the French Alps. As Spaceweather.com reported, the stunning sky scene she encountered featured an assemblage of eight remarkable atmospheric features.
Most of us have seen colorful rings around the sun before. Odds are that’s the 22-degree halo. It’s among the most common of the sun’s colorful optical phenomena, resulting from sunlight refracting through disorganized ice crystals and being split into its component colors. The 22-degree halo is always the same size and shape regardless of where the sun is in the sky.
But many of the other features require specific arrangements or orientations of crystals. Take sundogs, also known as parhelia (plural, or parhelion for the singular). Look for the two brighter patches intersecting the 22-degree halo on either side of the sun. Those form from nearly flat, hexagonal ice crystals that refract sunlight through their sides.
Those two sundogs fall on a larger strip of curved light that bisects the halo about halfway through. That’s a parhelic circle. Different parts of a parhelic circle are filled in by differing orders of reflections of sunlight.
Near the sun, it’s an external reflection, meaning sunlight bounces off the outside wall of an ice crystal and ends up somewhere else. Farther away, the sunlight enters the ice crystal, striking the wall from the inside before exiting through another face. On occasion, up to five or even more internal reflections can occur, illuminating parts of the halo most distant from the sun.
“Refraction, separating the sunlight into colors, occurs too,” explained physicist Les Cowley, who specializes in atmospheric optics. “The circle stays colorless because the refractions partly cancel and in any case the colors overlap to make [it] white again.”
Above and beyond the sun, columns of light — aptly named “sun pillars” — are visible. Sun pillars form similarly in some senses to parhelic cycles, but this time, the reflection occurs off the top or bottom of the horizontally oriented hexagonal platelets. They act like millions of little mirrors above and below the sun, reflecting sunlight toward the observer and causing a column of light to appear.
Then come the more unusual features. Like the upper tangent arc. Those require long hexagonal prisms oriented nearly horizontally. And based on the shape of the arc, we can estimate the sun’s elevation angle as having been close to 20 degrees above the horizon.
The Parry arc, likewise, requires similar circumstances. Parry arcs, however, feature the long hexagonal columns oriented both horizontally and with their top and bottom faces essentially flat. The type of arc that results is further complicated by the elevation angle of the sun.
Highest up, you have the circumzenithal arc and the superlateral arc. The former are caused by the same flat ice crystals that form sundogs. Circumzenithal arcs can form only if the sun is less than 32.3 degrees above the horizon. Superlateral arcs, like the one prominently visible in Bidois’s photograph, are often confused with larger circular halos (the 46-degree halo, which is much rarer). Superlateral arcs, however, develop when sunlight takes a path through the “ends” of a columnar hexagonal ice crystal and exits through a side face, or vice versa.
So putting this all together, how can you, too, spot a sky this colorful and eccentric? The bottom line is that you need some well-structured, organized ice crystals.
In this case, we can speculate that they may have been partially manufactured ice crystals. Many ski resorts produce their own snow, and often what’s produced ends up being a little bit “better” than what nature winds up with. Of course, displays like this still can — and occasionally do — occur fully organically.
In the case of Bidois’s photo, you’ll notice that there aren’t any visible clouds in the sky. Yet some sort of airborne ice is still needed to paint the display. The culprit is likely “diamond dust,” or clear-air ice crystals that drift aimlessly through the air on very cold days. When the air slightly above ground level is a touch warmer and more humid, it can mix with colder air down below and cause fine ice crystals to whimsically precipitate out of the air. This is most common in polar regions and high altitudes, but any place that drops sufficiently below freezing has a shot at enjoying the confetti-like nature of diamond dust.
Winter is the best time of year for halos and other optical phenomena, so be on alert these next few months for sky scenes of your own. And be sure to snap a pic! You might just help discover something new.