As most people know, the Northeast has been pelted by snow this winter. It seems like I haven’t seen grass in a few months, and navigating Lehigh’s mountainous campus is treacherous with all the ice and slush that persistently remain on sidewalks and walkways. One can only hope that a ski lift is in the works for the future, my meal plan is quickly running out trying to keep all the sled dogs fed. Needless to say, the snow, although scenic, is an absolute hassle. But what do you really know about snow? It’s white, it’s cold, it falls from the sky… As a chemical engineer, I wanted to take a look at the scientific makeup a snow and discover the chemistry behind the wet and cold nuisance. I reviewed several articles online and uncovered some interesting facts and tidbits about snow.
Snow is formed in clouds when liquid water freezes into ice (<32 deg F). Apart from temperature, air currents, humidity, and dust and dirt particles all influence the shape and size of snowflakes. If dirt particles are present in a snowflake, it will make the snowflake heavier and possibly crack the crystal. The shapes of snowflakes are determined mostly by the temperature and height of the clouds. Generally, six-sided hexagonal crystals are shaped in high clouds; needles or flat six-sided crystals are shaped in middle height clouds; and a wide variety of six-sided shapes are formed in low clouds. Also, colder temperatures produce sharper tips and may cause snowflake arms (dendrites) to grow. Snowflakes form faster in colder weather, causing the shapes to be more intricate and defined.
Snowflakes are symmetrical due to the intermolecular forces. Water molecules form weak hydrogen bonds with one another, these ordered arrangements result in the symmetrical shape of snowflakes. Water molecules align themselves during crystallization to maximize attractive forces and minimize repulsive forces. The molecules are arranged to fit spaces and maintain symmetry.
I’m sure everyone’s heard that no two snowflakes are exactly identical. Based on my sources, this is kind of true. On a molecular level, a snowflake will not have the exact same combination of water molecules, spin of electrons, and isotope abundance of hydrogen and oxygen. Given that the number of snowflakes to have fallen over the history of the world can be considered infinte, it is likely that two snowflakes matched in history. However, the likelihood of a person finding two identical snowflakes is basically impossible. Snowflake structure is constantly changing due to environmental conditions.
One of the main questions I had was; Why is snow white? It turns out that snowflakes have so many light-reflecting surfaces that scatter the light into all of its colors, causing it to appear white. Light sources all have color (eg. sunlight, incandescent, etc), so the human eye automatically subtracts the tint from scattered light, causing snow to appear white. I still don’t quite understand this idea, so I’ll do a little more research.
Possibly the most interesting piece of information I found was about colored snow. Now, I’m not talking about pouring blueberry syrup on a snowcone, but rather snow falling from the sky as a color other than “white”. The growth of algae is the most common cause for colored snow. Watermelon snow, which is red or green, is common in alpine regions worldwide. It also has a sweet scent reminiscent of watermelon. The cold-thriving algae contains photosynthetic chlorophyll which is green, and a secondary red carotenoid pigment (astaxanthin). Astaxanthin protects algae from ultraviolet light and absorbs energy to melt snow and provide the algae with liquid water. It’s remarkable how plants adapt to live in any climate, even in snow. There have also been reports of red, orange, brown, gray, and black snow throughout the world. This colored snow is attributed to dust, sand, and pollutants in the air. Anytime there is a spill or accident, the chemicals in the air may become incorporated with the snow. Remember not to eat the yellow snow though.
It was extremely interesting to learn about snow. Next time I’m sliding and slipping down the hill at Lehigh, I might consider the remarkable nature of each snowflake and the uniqueness under my wet boots.