Tuesday, February 10, 2015

The Science of Snowflake Shapes

TOS_snowflakesWho 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 crystal becomes too heavy to stay aloft in the cloud, and it begins the descent to earth, usually with many others like itself. 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 shape. 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: wellborn@nhcf.org

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The Adirondack Almanack publishes occasional guest essays from Adirondack residents, visitors, and those with a biding interest in the Adirondack Park.

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3 Responses

  1. Terry says:

    Good article! Try ‘googling’ a Jericho, VT gentleman named ‘Snowflake’ Bentley and be amazed at his skill of snowflake photography. Mr. Bentley (referenced in this article) was a 19th century marvel!

  2. Curt Austin says:

    In studying to become a metallurgist, I attended courses devoted to solidification and crystallography. It is all very fascinating! And of great practical consequence: did you know that jet engine turbine blades are directionally solidified so that they become single crystals with orientations? For those still awake, I’ll add some details about snowflakes.

    A water molecule strikes a bit of solid – does it stick, or just bounce away? (Or to be more precise, how long before the violence of thermal vibrations shakes it off?) It depends on the surface of the solid – how the solid molecules are arranged. This depends on the orientation of this surface with respect to its crystal structure. Ice has a hexagonal structure – there is a base plane, and six equivalent directions within that plane. Some of the crystallographic planes are more sticky than others – the surfaces at the tips of the spikes have the sticky orientation. Voila, snowflakes are flat (the base plane) with an arrangement of spikes at 60 degree angles.

    Another factor is that a molecule that lands on the solid must either shed the latent heat of both vaporization and solidification (unusually large for water), or it can’t stay. The need to dump this heat into the surrounding air also encourages a spiky structure – like fins on a radiator.

    I’m no expert on snow flakes, specifically. There are probably other factors in play. I don’t know why the left side of a snowflake is the same as the right side, or exactly why no two are alike.

    • Curt Austin says:

      Oops, HTML code and crystallographic directions do not mix – let’s try “crystals with <001> orientations”. Should be an “001” surrounded by angle brackets.