Anyone who has looked closely at a snowflake under a magnifying glass, or even with their naked eye, has an appreciation for the intricacy and delicacy of these frozen ice crystals that descend from the sky. Exactly how do they form and why do they assume the shapes that they do?
According to physicist Kenneth Lebbrecht, in his book The Snowflake: Winter’s Secret Beauty, snowflakes and snow crystals are made of ice. As its name implies, a snow crystal consists of a single crystal of ice. Snowflake is a general term that includes all shapes and combinations of snow crystals. A snowflake can be a single snow crystal, or a conglomerate of crystals.
A snow crystal is not a frozen raindrop. When raindrops freeze, they are referred to as sleet; the individual particles of ice lack the intricate patterns of snowflakes. Rather, snow crystals form when water vapor in the clouds condenses directly into ice. As more vapor condenses, the ice crystal grows and develops, creating elaborate patterns.
There is a sequence of events in the formation of a snow crystal. Evaporation from the ocean, lakes and streams, as well as the transpiration of plants and the expiration of animals puts a large amount of water vapor into the air. When a mass of air cools, the water vapor it contains condenses out of it. In summer, when this occurs next to the ground, we refer to the condensed water droplets as dew. When the air high above the ground is cooled, the water vapor condenses onto particles of dust, forming clouds full of water droplets. In winter, the individual water droplets start to freeze around 14 degrees Fahrenheit. They don’t all freeze at once; gradually the water droplets surrounding the particles of ice evaporate into water vapor which then condenses onto the ice crystals, growing snow crystals.
Many snow crystals begin as hexagonal prisms – flakes with smooth facets, or sides, arranged in a hexagonal shape. “Branches” then sprout at each of the six corners of this hexagonal crystal and as the surrounding water vapor condenses on them, they grow. Because the entire crystal passes through the same climatic conditions, the branches tend to grow in a similar pattern at a similar rate, creating the six-pointed star-shaped crystal, or stellar dendrite, that we are familiar with. Many shapes, including columns, plates and needles, are formed. Humidity, and particularly, temperature, affects the pattern of growth. Snow crystals tend to form simpler shapes when the humidity is low, and more complex shapes at higher humidities. Even so, the majority of snowflakes are not symmetrical. Within a given cloud, different snowflakes are blown in different directions, encountering different temperatures, which results in slightly different shapes. Thus, no two snowflakes are identical.