Halos in the Sky the Work of Ice Crystals

The word spread quickly around the Geophysical Institute on a recent Thursday: Drop what you're doing and go to the roof. Those who made the trip were rewarded with seeing crystal art in the sky: two bright spots on either side of the sun, a faint halo circling the sun, another halo farther out, and two upside-down rainbows above the sun. Even scientists who have worked at the institute for decades were voicing appreciation for the rare display.

The display was caused by sunshine and high, nearly transparent clouds, according to Walter Tape, who shot three rolls of film trying to capture the halo. A math professor at the University of Alaska Fairbanks, Tape has studied the arcs in Alaska and in Antarctica, where the displays occur frequently. "The crystals are beautiful, the halos are beautiful, and the math and physics that link them are beautiful," Tape said.

The two white spots decorating either side of the sun are called sun dogs, also known as parhelia, Greek for "from the sun." The lower upside-down rainbow that appeared above the sun is known as the upper tangent arc, and the brilliant version that appeared higher in the sky is called the circumzenith arc.

Both sun dogs and the circumzenith arc are created by the same ice crystals, Tape said. The crystals, shaped like thin sections of tiny pencils, act as prisms that separate sunlight into different colors. Tape thinks the six-sided crystals that created the recent display were part of cirrostratus clouds that were almost invisible. A call to Glen Woodall at the National Weather Service confirmed the presence of the hard-to-see clouds at about 25,600 feet.

For some reason, the best displays of halos and sun dogs in Fairbanks seem to occur in the springtime, though Tape said researchers don't yet know why. He also said the best displays in the world occur over the South Pole, for reasons that again remain a mystery.

"The halos are giving us some clue that the Antarctic atmosphere is special," Tape said. "But we haven't spent the same amount of time observing them in the north yet."

When he sees such displays in Alaska, Tape knows there's usually more to the show than what's going on near the sun. He said he performs an about-face to look in the opposite direction of the sun and is sometimes rewarded with a different vision, as he was recently. On the opposite side of the sky, he saw a white spot, known an the anthelion, along with other exotic halos. Later that night, he also received calls from people who saw a ring around the moon, caused by ice crystals that create clouds lurking in near-transparency about 10 miles above Earth, where passenger aircraft fly.

Tape, who is author of the book Atmospheric Halos, said he'd like to see photos of other halo events, especially those taken from the Arctic coast and other far-north places. His phone number is (907) 479-0156.