Jack Kwok, age 12, of Montreal, Quebec, Canada, for his question:

Can two snowflakes ever be identical?

Certain experts like to tell us that no two snowflakes are exactly alike. But Jack suspects that there must be a limit to their possible patterns and therefore their lacy designs are repeated  at least once in a while. And in theory, Jack is correct.

However, no matter hoc. many snowflakes are captured, preserved and examined under microscopes, no two are identical, or likely to be. Each one is an original arrangement of delicate ice crystals.

The problem here is numbers, astronomical numbers of submicroscopic ice crystals and infinitesimal pockets of air. These are the building blocks used to construct the almost infinite possible patterns of a snowflake. Ice, of course, is the solid crystal form of water. And every crystal must build itself on the form of its basic molecule. The shape of the water molecule ordains a six sided crystal and ice formations are limited to hexagonal designs.

One might suppose that this imposes a very limited number of possible patterns. But this neat idea is foiled by the multitude of miniature crystals used to build a snowflake, plus the uncountable air pockets of assorted shapes and sizes that are incorporated into the lacy arrangement. The number of possible designs is out of sight and a search for two identical snowflakes would be hopeless.

However, in theory there must be a limit to the number of possible hexagon arrangements. Nobody knows how many flakes fall during a mayor blizzard, or during a polar winter. It is possible and perhaps probable that a couple of those flurrying feathers chance to repeat the same pattern. It is not likely to happen often, because their multitudinous variations are governed by very fickle weather conditions in vast expanses of air.

At high altitudes, gaseous water vapor tends to forgo the usual three step procedure of condensing to liquid droplets that freeze at zero degrees centigrade.  Instead, it waits to become supercooled at air temperatures of minus 41 degrees centigrade or lower. Then, the gaseous vapor freezes directly to solid ice. This two step process is called sublimation.

The larger ice crystals that sublimate aloft may measure 1/50th of an inch. They may be needles or columns, prisms or plates and all are six sided hexagons. Multitudes of these assorted units are used to construct a single snowflake. Moment by moment, their construction work is modified by air and vapor pressure, by changing winds and temperatures.

A snowflake tends to be larger than a raindrop that contains an equal amount of moisture. It's airy crystal falls more slowly through perhaps six miles of air to the ground. On the way, it passes through levels of various weather conditons. Frosty layers tend to cause compacted flakes of dry, granular snow. Crystals that fall through warmish layers of air aloft, clog together in large, loose gobs and flurry to earth like fluffy white feathers.