Skywriting in the Clouds

The winter season in San Diego County is accompanied by fairly frequent instances of unusual optical effects in the sky. Many are associated with high clouds consisting of microscopic, hexagonal ice crystals which bend or reflect sunlight or moonlight, creating interesting geometric patterns of light overhead. These patterns can take the form of haloes around the sun or the 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.

Cirrus clouds, which appear wispy or feathery and lie at altitudes of around ten miles, will likely sweep over our area many times during the next two months. They are often the precursors of storms which either fortunately come our way in a day or two, or unfortunately pass north of us, leaving our landscape dry and thirsty. When the sun or a bright moon lies behind cirrus clouds, a circular halo of light with a radius of 22 degrees may appear, with the sun or the moon at its center. The tiny, hexagonal ice crystals in the clouds behave somewhat like 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.

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, 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.

Whether you’re pounding the city pavement, roaming through your favorite canyon, or climbing your favorite peak, take the time to cast an eye upward now and again, especially whenever wispy cirrus clouds grace the azure sky. You might see something magical.

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