Nearly everyone at some point has witnessed a rainbow-- curving majestically against a retreating storm, or dancing in the spray of a lawn sprinkler. Likely, you know that a rainbow is sunlight refracted through countless suspended water droplets, acting like miniature prisms. You may even know that a rainbow can be double, twinned, or monochrome. But the refractive magic of sun and sky does not end there. Enter the ethereal realm of halo phenomena.
Halos arise from sunlight refraction through microscopic ice crystals, usually aloft in cirrus clouds. The crystals have a variety of hexagonal forms-- thin plates, slender rods, elongated pyramids --imbued with remarkable optical properties. In combination with crystal orientation, uniformity, and motion these properties can produce breathtaking effects.
Take the resplendent 22° halo, iridescently encircling the sun. It stems from sunlight refracted through opposing edges of plate crystals. Often, it is decked with bright bead-like nodules called parhelia (or sundogs). At times, halo and parhelia are interconnected by an extensive pearlescent band-- the parhelic circle. Usually incomplete, the circle occasionally closes on a faint sundog-like anthelion in the opposite sky.
Then there is the ghostly 46° halo, concentrically dwarfing the 22° halo. Here, plate crystals refract sunlight perpendicularly, casting a wider but dimmer apparition. Usually absent parhelia, the 46° halo has its own appurtenances-- the crescent crown of the circumzenithal arc, the fiery sash of the circumhorizontal arc.
When rod crystals enter the refractive mix, additional features can arise-- vertical infralateral arcs glancing a 46° halo, v-shaped tangent arcs pinning a 22° halo, the convex capstone of a Parry arc. The sweeping supralateral arc may also occur, closely resembling the 46° halo in position and curvature, but never forming a complete circle. With pyramidal crystals in the mix, the narrow 9° halo may make a tight bullseye of the sun, with a dart-like Moilanen arc aimed towards its center.
The rarest phenomena include stringy Lowitz arcs festooning a sundog, and the spectral Kern arc (actually a circle) corralling the zenith, among others. Virtually any combination of phenomena can appear in the heavens, as history has portentously noted.
Fun facts: people standing side-by-side do not see the same halo. A unique set of refracted light rays converge at each person’s eye position, producing a halo exactly visible to that person alone. And yes, the moon can also conjure bows, halos, and dogs of its own, using the same ice crystal optics.
Day or night, the sky harbors grand visual magic. When you catch it mid-trick, count yourself a lucky person!
You’ve been listening to Western Slopes Skies, produced by the Black Canyon Astronomical Society and KVNF Community Radio. This feature was written and voiced by Michael T. Williams.
