Urban Jungle

When you're standing in a blizzard, it's hard to imagine that no two snowflakes are exactly alike. Surely at least one duplicate is possible.

"The odds of it happening within the lifetime of the Universe is indistinguishable from zero," writes Caltech physicist Kenneth G. Libbrecht on his Web site SnowCrystals.com.

Water can crystallize out of a snow cloud in 35 forms, including needles, bullets, cups, columns, graupel and flat, lacey snowflakes. But all crystals start out as microscopic hexagonal prisms (figure 1). If you looked at enough of these tiny crystals under a microscope, you would probably find some that look alike.

The big, intricate flakes, however, are each visibly unique. If conditions in the snow cloud are favorable, the tiny hexagonal starter crystals will develop elaborate branches and textures (figure 2), varying with wind, humidity, temperature and chemical impurities. A small percentage of the flake's water molecules will be composed of different isotopes of oxygen and hydrogen, which also affect the design.

Each branch of a snowflake grows independently of the others, but identical conditions will make branches grow in the same manner. However, "the vast majority of snow crystals are not very symmetrical," Libbrecht writes. "Don't be fooled by the pictures."

The images at right picture fairly symmetrical snowflakes. Libbrecht suggests that an observer might discern 100 different features on each snowflake. "Since all those features could have grown differently, or ended up in slightly different places," the number of possible arrangements for each snowflake approaches 10¹⁵⁸, or a 1 followed by 158 zeros. "That number," he says, "is about 10⁷⁰ times larger than the total number of atoms in the entire universe!"

SOURCES: National Solar Observatory, "The Physics Factbook"

In 1885 Vermont farmer Wilson A. Bentley pioneered a process for photographing snowflakes. The patterns at left are derived from a few of the more than 5,000 images he recorded.

Images courtesy of The Jericho Historical Society, Jericho, Vermont.