"Oh, the weather outside is frightful, and the fire is so delightful, and we've got no place to go - so, let it snow, let it snow, let it snow..."
With the beginning of winter upon us, have you ever stopped while shovelling the driveway and wondered why no two snowflakes are the same?
Snowflakes and other forms of precipitation grow in the upper atmosphere from the condensation of water vapour upon tiny particles and aerosols. That is, as you and I and all living things, including the plants around us, respire, we introduce water vapour into the atmosphere. The oceans, lakes, and rivers also contribute as they are continually evaporating, producing water vapour that is borne aloft by the air. Even the land itself contributes as moisture is lost from exposed soil. Water vapour is an integral part of the water cycle and pretty much everything contributes to the process.
But all of this moisture or water vapour stays as a gas in the air provided the temperature does not drop below a critical point and there are no sites for "nucleation". On a warm day, you can't see the water vapour in your breath although you can see it if you blow onto a cold surface, such as your glasses. Warm weather means that atmospheric temperature is above the critical point.
Even on a cold day, for water vapour to drop out of the atmosphere, nucleation sites must be present. (Nucleation sites are any small particles of dirt or dust or particulate matter raised into the upper atmosphere by the winds as they blow across the earth.)
When the temperature is below the critical point, water vapour condenses into the liquid (rain) or solid (snow) state on these "nucleating" particles. However, each water molecule arrives at a different time and overall the water molecules arrive randomly at the surface of the growing precipitation. The growth of a snowflake reflects these vagaries. That is, the water vapour arrives at different rates and times, depending upon such things as the wind, the temperature, and the amount of moisture in the air and each combination of parameters generates a different type of snowflake. There are six generally recognized forms and not all of them are the beautiful six-sided lacy flakes that we think of when we think about snow.
As each water molecule hits the surface of the growing flake, it is stuck in place. Frozen in ice, the water molecules can't shift around. The random arrival of water vapour means that the final shape of a snowflake can never be predicted nor reproduced. Hence, no two flakes appear the same.
More importantly, if you look at a snowflake under a microscope or with a magnifying glass, the incredible beauty of these tiny white bits of ice are revealed. And although no two flakes are the same, they do exhibit some common features. The most important is that they all have six-sided or hexagonal symmetry. The tiny arms and branches, the very shape of the ice crystals themselves, are six-sided.
But why? The shape is due to the structure of the water molecules from which the ice is created. Water is composed of two hydrogen atoms and an oxygen atom. It is slightly v-shaped with the hydrogen atoms extending up the arms of the "v" and the oxygen at the point. Another feature of the oxygen atom is that it has two pairs of electrons sticking out from it, in addition to the hydrogen atoms. These make another v-shape but pointed in the opposite direction and twisted ninety degrees with respect to the hydrogen. An "X" twisted around the middle would be a good model. The whole thing is shaped like a tetrahedron - a four-sided shape with regular equilateral triangular faces.
In the formation of ice-crystals, it is the equilateral triangular faces that are important as six join together to produce a perfect hexagon. This atomic-sized hexagon is repeated countless times in producing the snowflake that we see. A single snowflake will contain billions upon billions of these tiny hexagonal rings. These hexagons at a molecular level generate the macro-scale six-sided symmetry of snow.
All of this results in an underlying pattern of crystalline beauty that produces our magnificent winter wonderland or "that damn snow", depending upon whether or not you're the one shovelling the driveway! In any case, winter is upon us.
