Look for ice-crystal halos, pillars, and colored arcs in San Diego's fall and winter skies.

When zillions of photons dart among billions of spinning, microscopic ice crystals in cirrus clouds high above, order may gel amid apparent disorder, creating a bevy of optical effects in the atmosphere. These effects include halos around the sun or moon, luminous pillars of light that appear before sunrise or linger after sunset, and curious mini-rainbows -- "sun dogs" -- that sometimes bracket the low-angle sun.

Ice crystal sky effects

Cirrus clouds, which appear wispy and lie at altitudes of around ten miles high, will likely pass over our area many times during the next five months. When the sun or a bright moon lies behind them, a circular halo of 22 degrees in radius may appear, with the sun or the moon at its center. Cirrus clouds consist of tiny, hexagonal ice crystals that behave somewhat as simple prisms do, refracting (bending) light from a point of origin through an angle of 22 degrees. If the ice crystals are randomly oriented within the clouds and the clouds are uniformly thick around the source of light (either sun or moon), then the halo will appear to be about the same brightness all the way around. In many parts of the world these halos -- and indeed cirrus clouds themselves -- are indicative of rain or snow to come, following a pattern of thickening cloud cover. Here in San Diego, this is often not true, as storms usually bypass us to the north.

Sun dogs (a.k.a. mock suns, false suns), which look like small, color-fringed spots some 22 degrees left and 22 degrees right of the sun, can sometimes be observed from the San Diego area. Their origin, as worked out by physicists, is once again due to sunlight refraction, but in this instance through ice crystals that have a preferred orientation of spin.

On rare occasions (from our latitude, at any rate), near the time of sunset or sunrise, you may chance to see a sun pillar, a vertical shaft of light floating amid thin, high clouds above the position of the sun. This is not due to refraction, but rather we are seeing the reflected glint of sunlight on the bottom surfaces of countless plate-like ice crystals that are falling horizontally through still air.

Dozens of rarer and more exotic ice-crystal effects have been carefully noted and explained in the scientific literature. Among them are "circumzenithal" and "circumhorizontal" arcs, which look like strange, horizontal segments of a rainbow, up near the zenith (the straight-up point) or down near the horizon. I've seen only one of each, despite thousands of days spent outdoors.

So, when roaming about through the outdoors this fall and winter, cast an eye upward now and again, especially whenever wispy cirrus graces the azure sky. You might see something magical.

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