Let’s start with an introduction on snowflake formation. As you’re probably aware, evaporation from lakes and rivers, transpiration of plants, and human and animal exhalation send water vapor up into the atmosphere. That water vapor gathers in clouds.
Water molecules in clouds engage in a complex dance of phase changes. Variables such as temperature, pressure and even the presence of dust can affect snow formation. When it gets cold, the gaseous molecules want to enter the liquid phase, but it’s much easier for them to do so if they can find a solid on which to settle. This phenomenon is known as nucleation, and it can be observed right here on Earth.
Try microwaving water in a glass. If your glassware is exceptionally clean, the water will get very hot — hotter than 212 degrees — and still not bubble. But if you place a chopstick or other rough-surfaced item into the water, it will suddenly boil furiously, as the water moves from liquid to gas. In a cloud, dust serves the same purpose: It provides a surface on which water can change phases (in this case, from gas to water).
A cloud is essentially a mass of water vapor molecules that have found dust particles and condensed into water. As a cloud cools, a few of the droplets begin to freeze, while others resist turning to a solid. Some even evaporate back into their water vapor form. When the water vapor molecules come into contact with a frozen droplet, they freeze into a solid. Skipping the liquid phase is crucial to the snowflake process: It’s the difference between getting an ice droplet and a flake.
Infant snowflakes show little variation in their hexagonal design. If the process stopped here, all snowflakes would be visibly indistinguishable, 10-molecule structures. (The word “visibly” has to be understood metaphorically here. You couldn’t see a 10-molecule snowflake, even with standard optical microscope.)
But it doesn’t stop there. Parts of the growing snow crystal have rough edges, or bonds, dangling off their sides, which are better at attracting other water molecules than the smoother parts of the hexagon. As more water vapor molecules settle on these bonds, the rough edges become a relatively large protuberance with its own rough edges, which attract more water molecules. Through this process, the rough edge becomes a branch of the crystal. Eventually the branches develop their own rough edges, which develop into sub-branches of the original branch, and so on.
The temperature and humidity also affect how new water vapor molecules bond to the growing snowflake. Warmer, drier conditions tend to create solid plates and prism shapes — the kinds of snowflakes that look boring to the layperson. As conditions in the cloud get colder and more moist, the beautiful winter-wonderland-type snowflakes become more common. Scientists refer to these shapes as dendrites and sectored plates. The same conditions can also create needles, which are long, thin snow crystals that don’t look like flakes at all.