“When a snowflake melted, that design was forever lost. Just that much beauty was gone, without leaving any record behind.” W.A. Bentley, 1925
Snow is magical, renewing our sense of wonder, transforming a barren landscape into a winter wonderland, and drawing us outdoors. When we look closely at falling snow, we marvel at the beauty of snowflakes. Each snow crystal is a unique, ephemeral work of art – yet its symmetrical, harmoniously balanced design is not random but created through a combination of physics, math, and chemistry.
Snow forms in clouds at temperatures anywhere between 32⁰F and -39⁰F. These clouds are full of microscopic water droplets, thousands of which could fit on the dot of an i. Mixed with these droplets are “snow seeds” – tiny particles of salt, dust, pollen, or airborne pollution. These snow seeds act like magnets, attracting the droplets, which adhere and freeze onto them. The chemical and physical nature of water causes these freezing droplets to form as microscopic six-sided symmetrical crystals.
While all snowflakes start out as these microscopic hexagonal crystals, temperature determines the specific type of snow crystal that will develop, and humidity levels influence the size and complexity of their designs. At certain temperatures vertical growth may occur, creating columnar crystals, while other temperatures produce flat crystals that grow primarily in horizontal planes. The trip from sky to earth takes a tiny crystal though many layers of air with different temperatures and varying amounts of moisture. All these combine to make each individual snowflake unique, as no two travel the exact same path through the sky.
Anyone who lives with snow has their own ways to describe and classify it, from children’s snowball snow to the skiers’ powder snow. Scientists classify snow based on crystal shape. Though some classification systems include dozens of different categories, the standard 1951 classification by the International Commission on Snow and Ice recognizes seven common shapes: needle, column, capped column, plate, stellar, spatial dendrite, and irregular crystals.
Needle crystals are long slender columns with points at either end. They often stick together in bunches and can sting your face and shatter when landing on hard surfaces.
Columns are tiny six-sided tubes of ice often hollow due to air space inside. They often form in high, cold cirrus clouds with low moisture. In winter, these wispy cirrus clouds are almost solely made of column crystals.
Capped columns are combination crystals. Their columns form in high, dry clouds, but as they fall to earth they pass through warmer, moister clouds. There they grow horizontally at their tips, forming plates at opposite ends.
Plate crystals are six-sided hexagons with flower-like patterns within. Increasing moisture causes the crystal to expand outward, forming fancier plates with petal-like projections on their six sides.
Stellar crystals, also called stellar dendrites, look like snow stars, with six points radiating out from a center, like spokes on a wheel. The star-like points get more elaborate with increasing moisture, and nearby stellar crystals easily get entangled. They often fall to earth in clusters.
Spatial dendrites are the three-dimensional versions of stellar crystals with projections growing upward at right angles from their flat surfaces.
Finally, some of the snow crystals do not fit into any of these categories and thus are referred to as irregular or asymmetrical crystals.
Of course, precipitation other than snow can fall in winter. Freezing rain describes liquid raindrops that fall when temperatures at ground level are below 32°F. Usually freezing rain is associated with a warm front coming in above a cold front. The warm air up high allows raindrops to form. A relatively thin freezing layer of air (32°F and colder) trapped near the surface chills the rain so that the drops can be colder than 32° and yet remain liquid! When these supercooled raindrops reach earth, the rain freezes on contact with the below-freezing roadways, branches, and utility wires, coating everything in a glaze of ice.
Sleet is precipitation that falls as frozen raindrops. It typically forms when there is a layer of warm air sandwiched between two layers of freezing air (temperatures 32°F and lower). It starts out as snow in the top freezing layers, but as it falls through the warmer layer of air with temperatures above freezing, the snow crystals almost completely melt into raindrops. These drops refreeze to tiny balls of hard ice as they pass through the thick layer of freezing air below. These transparent ice pellets are frozen raindrops and bounce when they hit the ground.
If you have ever been out in a snowstorm, you’ve probably seen snow that looks like miniature popcorn mixed with snowflakes. That’s graupel – snow crystals and supercooled water droplets frozen together. Falling snow crystals (often stellar or plate) pass through a layer of liquid water droplets, which freeze onto the crystals and coat them with ice/rime. This freezing of droplets onto the crystal (called accretion) continues until the original snow crystal is no longer visible. Graupel appears opaque white, easily breaks apart, and often bounces as it lands. Graupel is lightweight with many small air bubbles throughout its structure.
Each snowfall contains different types of snow crystals. The predominant crystal type determines how much the snow adheres, builds up, and drifts once on the ground. Snow has different qualities depending on how much water, ice, and air it contains. Snow with high water content, mostly stellar and plate crystals that link together when compressed, is perfect for molding into snowballs and snowmen. On the other hand, shoveling is easy when a snowfall contains mostly columns and capped crystals, as each shovelful feels light and fluffy.
A snow crystal’s life is one of constant change, from its trip through the sky to its resting place in the snow bank. For once on the ground, the crystals immediately change again. The fine arms of stellar crystals evaporate, delicate surface patterns on plates disappear, and all snow crystals condense inward into small bits of ice. This change is called metamorphosis. Over time, the snows from each snowfall condense into layers, like several blankets piled on top of one another. Variable winter weather conditions change the texture of each. Slicing into a snow bank reveals its profile and tells its history, with each layer representing the change from delicate snowflake to ice particle and ultimately back into the water cycle.
Snow has a way of bringing out the kids in all of us – drawing us outside to play. It provides us with the medium to create anything from simple snowmen to elaborate sculpture. Yet all of these designs, just like the snowflakes that made them, are fleeting. So head outside and enjoy the snow.
Libbrecht, Kenneth. The Snowflake: Winter’s Secret Beauty. Stillwater, MN: Voyageur Press, 2003.
Libbrecht, Kenneth. A Field Guide to Snowflakes. Stillwater, MN: Voyageur Press, 2006.
Marchand, Peter. Life in the Cold. Hanover NH: University Press of New England, 1987.
Stokes, Donald and Lillian Stokes. A Guide to Nature in Winter. Boston: Little, Brown and Co., 1976.
Cal Tech Snow Crystals website: http://www.its.caltech.edu/~atomic/snowcrystals/class/class.htm