The intricate beauty of a snowflake is one of my greatest winter delights. It follows then, that the science of snowflakes has captured my imagination. “The physics of snow crystals” by Kenneth Libbrecht was a feast for my snow-focused curiosity (full citation at bottom). Libberecht mentions that one reason snowflakes are of scientific interest is that understanding their crystalline growth patterns may benefit industries also using crystalline material.
The article is full of snow science knick-knacks. He explains two of the great snowflake mysteries: why they are so varied and how they form such intricate patterns. To build on my Scientific American guest blog post, Winter Wonders: The Science of Cold, here is a bit more snow crystal formation, all attributed to Libberecht 2005.
On the dramatic variation in snowflakes (or snow crystals):
The two primary variables determining snow crystal size and shape are temperature and humidity. There are diagrams with temperature on the X-axis, air saturation on the Y-axis, and a spread of snowflake types occurring at each coordinate. The general pattern is this:
|Type of snow crystal|
|0 to -2º C||Small hexagonal plates|
|-2 to -10º C||Columns and needles|
|-10 to -22º C||Large hexagonal plates|
|-22º C and below||
Mixture of columns and plates
The typical six-armed flake falls into the hexagonal plate category. And when you seereally big, beautiful flakes, it’s likely the temperature is between -10 and -22º C (14 and -7º F) and that the air is highly saturated with moisture. The lower the humidity, the smaller the snow crystal will be.
Even with these known patterns, snow falling in the same winter day, hour and even minute will be highly varied. Snow crystals are hypersensitive to changes in humidity, temperature, and other factors, like airflow. Minute changes will create a slightly or possibly dramatically different crystal. The microclimate around each flake is responsible for its ultimate form.
On the intricate nature of snow crystal patterns:
Within the right temperature ranges, snow crystals will be flat and six-sided. And within those temperature ranges, if the air is holding a lot of moisture (e.g. 0.3 g/m3) and the air pressure is high, the six-sided crystals grow branched arms. This type of formation is called “dendritic (tree-like) growth.” After dendritic growth begins, the snow crystal can become very complex. Some of the smallest details on a snowflake are 1 to 10 micrometers wide, which is less than 1/10 the width of a human hair. These microscopic intricacies are incredible when seen by a scanning electron microscope. Several such images are publicly available.
From here, even if I got into the math-y details, scientific understanding of snowflakes falls short. Exactly how the crystalline branches form isn’t clear yet. There are still many unanswered questions in the science of snowflakes.
To make your own observations, or to simply experience the wonder of water vapor crystallizing into delicate patterns, I join Libberecht in inviting you “…to go outside with a magnifying glass or microscope during a light snowfall .” (858)
Libbrecht, Kenneht. 2005. The physics of snow crystals. Reports on Progress in Physics. 68: 855-895.
Image credits: Electron and Confocal Microscopy Laboratory, Agricultural Research Service, U. S. Department of Agriculture. More images here.