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Scientists Find Two Snowflakes Alike

June 14, 1988 GMT

WASHINGTON (AP) _ A cherished common belief may be in doubt: the assumption that no two snowflakes are alike.

Nancy C. Knight of the National Center for Atmospheric Research in Boulder, Colo., has discovered what may be the first matching set of snow crystals.

″My first reaction was to say ’that’s absolutely impossible,‴ said her husband, Charles Knight, also a researcher at the center.

But there the two crystals were, side by side, on a glass slide exposed in a cloud on a research flight over Wausau, Wis.

″One of the most quoted statements about snow crystals is that no two are alike, a bit of folk wisdom that is generally accepted even among those few regarded as experts in the subject ...,″ Mrs. Knight said in a letter published in the may issue of the Bulletin of the American Meteorological Society.

But, she continued, she found ″a striking example of two snow crystals which, if not identical, are certainly very much alike.″

″In many years of snow-crystal collection the author has seen no other examples of such crystals, nor are any given in the standard references,″ Mrs. Knight wrote.

″When you say no two are identical, that’s a sort of philosophical question; there have been an awful lot of snow crystals over the history of the Earth,″ Charles Knight said in a telephone interview. ″Especially with small ones, it is likely many were similar to one another. But these two are a paradox.″

Mrs. Knight is out of the country doing research on hail. The center at which she and her husband work is co-supported by the federal government and a group of colleges and universities.

Meteorologists prefer the term snow crystal to snowflake, reflecting the fact that these entities take many forms other than the popular six-pointed image. Snow crystals can be shaped like stars, columns, needles, plates or just lumps.

The pair of crystals photographed by Mrs. Knight are shaped like columns with vase-shaped hollow centers. They are tiny - only one-quarter millimeter (0.009 inch) the long way, slightly less on the short side.

Column-shaped snow is not really unusual, Charles Knight explained. ″When you go out and see real snow crystals you beigin to appreciate that those beautiful crystals in (picture books) are really quite rare.″

Because crystals tend to grow fastest at their edges, column-shaped crystals with hollows in the middle - called lacunas - are not unusual.

The two ″virtually identical″ crystals were collected on Nov. 1, 1986, while researchers were studying clouds.

The crystals were collected on a glass plate coated with oil, which was exposed to the cloud for 11 seconds at an altitude of nearly 20,000 feet. The plate was then kept cold until the airplane could return to the ground and the snow crystals were photographed.

The crystals ″don’t exist any more,″ Knight noted.

In order to develop such identical shapes, the two ″had to go through identical growth histories,″ accumulating freezing water as they pased through layers of cold air, he explained.

Because they are alike and were found stuck together on the slide, it almost appears as if they should have grown to their final shapes while stuck together, Knight said. But if they had grown while attached to one another, they should not each have a complete, symmetrical shape, he added.

″There has to be some resolution to this paradox,″ he said.

What’s the likelihood of finding more identical pairs of snow crystals?

″People should keep their eyes open,″ Knight said. Research in cirrus clouds such as those where the matching crystals were found has become increasingly popular in recent years, so more pairs of flakes could turn up, he said.

For those concerned about other common weather beliefs: Yes, lightning can strike more than once in the same place. Indeed, researchers have found that it seems to prefer some spots, usually trees on high ground.