While our coastal area doesn’t see as much snow as the more mountainous areas of North Carolina, snow is a fundamental aspect of winter in many regions. When you hear the word snowflake, you likely picture a symmetrical, six-sided crystal like the ones above; however, little was known about how complex and beautifully patterned snowflakes were until Wilson Bentley from Jericho, Vermont started paying close attention in the late 1800s1. Over a century later, we have more information about how snow forms, the conditions necessary for it, and can even grow designer snow crystals in a lab.
Bentley, pioneer of snowflake imaging, began his career at the young age of 14 with limited technology and tools: he would catch snowflakes, examine them under a microscope, and sketch their shape with a pencil before they melted away2. In his early twenties, Bentley took the very first photographs of snowflakes by attaching a camera to his microscope and experimenting to find the ideal photographic conditions. Getting clear images of snowflakes with turn-of-the-century technology was impressive and painstaking work. Though the residents of Jericho may have considered his hobby impractical, Bentley’s passion became a career resulting in scientific articles and speaking tours, and his images were published in the book Snow Crystals (1931)2.
A snow crystal is the more specific term for a single snowflake. The term snowflake is general and encompasses both snow crystals and clumps of snow crystals that form into a larger flake3. Snow crystals form when the atmospheric conditions allow water vapor to turn directly into ice, skipping the liquid step – a process called deposition. This generally occurs at atmospheric temperatures between 5°F and 23°F4. The formation begins when water vapor freezes around a tiny particle floating in the sky, such as pollen or dust.
All precipitation begins as snow high up in the clouds where temperatures are always low. It is only as the snow falls to the ground that it can change into other forms of precipitation such as rain or sleet. Snow may encounter layers of warm air that cause it to melt back into liquid water. As it continues to fall to the ground it may pass through another layer of air with temperatures cool enough to refreeze the water, resulting in sleet5. Sleet occurs when liquid water freezes into ice, forming small round pellets. Snow occurs when water vapor freezes into ice without becoming liquid first, forming crystals3.
Pictured above: The crystal structure of frozen water molecules. Image credit: Danski14, Wikipedia.
When water freezes, the molecules form a crystal structure in the shape of a hexagon. This hexagonal structure results in the symmetrical six-armed shape that makes up the classic snowflake3. As the snowflake travels through pockets of clouds with varying temperature and humidity, water vapor condenses into ice on the crystal, and the arms grow. The precise shape of the snowflake is determined by the path it takes through the atmosphere. Since all six arms of an individual snowflake travel the exact same path, they grow in the same way, leading to a symmetrical shape. Because no two crystals take the exact same path through the clouds, no two crystals are identical. Each and every one is distinct and unique.
In addition to the traditional six-sided snowflake, snow also comes in irregular shapes such as columns, needles, plates, and capped columns. The atmospheric conditions at the point of formation contribute to the shape of a snow crystal. Low humidity tends to form simpler structures like plates and blocks, while high humidity creates more complex branched crystals6. The next time you’re in a snow storm, take a closer look around and look at some flakes with a magnifying glass. You might be surprised at what you find.