Welcome to You Ask Andy

Randy Stanifer, age 12, of London, Kentucky, for his question:

What has every snowflake a different design?

Several years ago, researchers probed deep into the secrets of snowflakes. The laboratory conditions precisely duplicated those in which snowflakes form in the weathery atmosphere. The results were astonishing because the procedure did not follow the usual rules of ice and water. Scientists had to fathom a brand new set of rules to solve the mysteries of how snowflakes are formed and why they look like they do.

Feathery snowflakes are created from minuscule crystals of ice interlaced around minuscule pockets of air. We take it for granted that water freezes to ice at 0 degrees centigrade. Hence we would expect moisture in the air to form the basic ice crystals at this temperature. Then the snowflake building could begin. But the laboratory experiments revealed this is not the way it happens at all.

The moisture in the air is reluctant to become solid fragments of ice until the temperature falls several degrees below the normal freezing point of water. The tiny crystals needed to construct snowflakes may be no bigger than molecules    and they are formed in different shapes. All of them, however, build the six sided, or hexagonal, structure dictated by the shape of the basic ice molecule.

Ordinary bricks are cubic oblongs so they fit naturally together for building walls. The odd shape of the water molecule forms hexagonal crystals of solid ice. And these miniature building blocks naturally arrange themselves in hexagon patterns.

The shapes and sizes of the ice fragments vary as the temperature falls. At 3 degrees Centigrade below freezing, the moisture the air tends to form flat, six sided plates of ice. At minus 5 degfees centigrade it tends to form tiny crystals shaped like jagged needles. At minus $ degrees centigrade, hollow crystals are formed in six sided columns. When the air is super dry, the creation of this assortment of miniature ice crystals requires a lower temperature range.

A low hanging snow cloud is a turbulent mixture of air masses    cool and cold, moist and dry. This situation creates a wide variety of floating, flying ice crystals. When the temperature drops to around minus 12 degrees centigrade, these minuscule building blocks begin to team together and construct snowflakes. The construction work goes faster when the assorted particles can assemble around floating dust, fragments of salt and silica, mica and other minerals.

Snowflakes use a variety of ice crystals as they are wafted between different weather conditions in the cloud. This creates variety in the hexagonal assembly of crystals and air pockets. Countless crystals of assorted shapes and sizes are assembled while the growing snowflake is shifted from here to there. It is now likely that two flakes are built under identical conditions. It is not likely that two identical snowflakes fall, even in a huge blizzard.

Snowflake assembly also varies with temperature. Lab experiments showed that the growth of a perfect six sided flate needs calm, gently cooling air and plenty of time. Sudden cold drafts cause lopsided structures. In the atmosphere, a growing snowflake wafts through countless drafts and selects its crystals from a countless assortment. Identical twins are not impossible, but so far nobody has found a pair.



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