By Barbara Mackay
Who hasn’t marveled at a lacy snowflake coming to rest on a jacket sleeve? Do you wonder how it could survive the fall to earth in one piece, or if it’s really true that no two snowflakes can look exactly alike?
A snowflake begins high up in the clouds, not as a snowflake but as a small particle of dust, salt, or ash. When a cloud cools below 32 F, some specks of water vapor freeze onto the particle. As it moves through the cloud, the particle absorbs additional water vapor, building up microscopic layers of ice. When water molecules freeze, they bond together in a way that forms a six-sided ice crystal.
Eventually, the ice crystals become too heavy to stay aloft in the cloud and begin the descent to earth. This is the precipitation we call snow. A snowflake can be a single ice crystal, but it’s usually a cluster of crystals, often two hundred or more, all symmetrically connected. The common feature of all ice crystals, and thus all snowflakes, is the hexagonal structure of branches, short stub-like arms, or sides.
Snow crystals are categorized by their general shapes. There is the familiar intricate star-shaped snowflake, called a stellar dendrite. Spatial dendrites also have branches, but they are more untidy and dissimilar. Plates have just the suggestion of arms growing at the six corners of the hexagon. Other common types are needles, columns (which look like short, unsharpened pencils), and capped columns (which look like columns with graduation caps attached to the ends). Eighty snow crystal varieties have been formally identified, catalogued, and labelled by scientists who study the world of winter. Two books with exquisite photographs are “The Snowflake: Winter’s Secret Beauty” (Libbrecht and Rasmussen) and “Snow Crystals” (Bentley and Humphreys).
Different cloud temperatures and humidity levels determine the snow crystal’s initial size and shape. Thin, plate-like crystals grow when temperatures are close to the freezing point. At cooler temperatures, needles, columns, and dendrites appear. Additionally, higher humidity produces more intricate designs, while simpler forms of plates and columns occur in drier air.
Finding a perfectly intact snowflake is a treat. In a thirty-minute free-fall, snow crystals are tossed about by the wind, encounter different temperatures and levels of humidity, and get in each other’s way. Pieces may break off or melt, or crystals that collide might stick together. So, can two snowflakes be exactly alike? Only if they attract water vapor in a cloud in the exactly same way, fall through exactly the same temperature and moisture conditions, and swirl in identical aerial dances. Perhaps a statistician could give the odds, but I rather enjoy conducting my own research during a snowstorm.
It is easy to collect snowflakes for your own enjoyment and study. Keep a sheet of stiff black paper in the freezer, ready to grab when flakes start falling. Stand out of the wind and let snowflakes land on the paper. Then retreat to a protected area that is cold, such as a garage or covered deck. Use a magnifying glass to study the individual flakes. The hardest part is not breathing warm air on them!
You can also preserve snowflakes on a glass microscope slide. Keep the slide in the freezer and a can of clear acrylic spray in the refrigerator. When you are ready to go outside, put on gloves (you do not want the slide to warm up in your hands), spray the acrylic onto the slide, and dash outside. You can hold the slide in your outstretched hand or place it on a level spot. Let a few flakes fall onto it, then place it in a protected spot outside for a few hours. The snowflakes will freeze into the plastic spray, which also freezes, creating a permanent impression of your snowflakes. Under the microscope you will be able to see tiny parts of snowflakes that you missed with your own eye.
Barbara Mackay is a teacher and naturalist who lives in northern Vermont. The illustration for this column was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: firstname.lastname@example.org