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Last year, we explored the properties of various densities of water in a post looking specifically at the density of snow, which is an effect of the density of the ice in the snow itself, the shape and structure of the snow crystals, and how closely the crystals pack together when they hit the ground. This year, we’re getting into the nitty gritty (literally!) of snowflake formation to give you a new appreciation for this cold-weather wonder.
While you might consider snowflakes simply to be frozen water, if you compare them with the frozen ice in your freezer, you’ll realize that they’re very different. Unlike ice cubes, which form when liquid freezes into a solid, snowflakes form when water vapor turns directly into ice during a process called deposition.
Most snowflakes develop at or just below freezing—0º Celsius or 32º Fahrenheit—because If the air gets too cold, a cloud won’t hold enough water for anything to precipitate out. Snow is able to form in cooler environments, but the colder the air gets, the less moisture will be available to make a snowflake.
In fact, a cloud’s air has to be supersaturated with moisture for a flake to form, meaning there is more water in the air than would normally be possible. When there is too much liquid water in the air, a cloud will try to rid itself of the excess. Some of that excess can freeze, transforming into crystals, which then slowly fall to the ground as snowflakes.
Similar to the way pearls form inside mollusks with the help of a tiny irritant, water droplets can’t freeze without something to attach to. Even when the air temperature is below freezing, water droplets will remain liquid until they connect with a solid object on which they begin to build their crystalline structure. Usually, that will be something like a grain of pollen or a dust particle; smog-like aerosols or organic compounds released by plants can also trigger the growth of a snowflake.
If you look closely at snowflakes, you’ll notice they have six sides. To understand why, we need to look even closer. The six symmetrical sides of snowflakes reflect the internal order of the crystal’s water molecules. A molecule of water is made of two hydrogen atoms bound to an oxygen atom.
Alone, due to polar covalent bonds, the three atoms combine to form a “V” shape, caused by the fact that the three atoms unevenly share electrons with each other. But when multiple water molecules find themselves near each other, because opposite charges attract, they begin to rotate so that a negative hydrogen atom realigns itself toward a positive oxygen atom. The shape that results is hexagonal.
As they fall through the air, the growth of snowflakes depends on atmospheric conditions such as humidity and temperature, and changes in these conditions can affect how they grow. However, conditions at each of the six arms of a snowflake remain the same; therefore, the snowflake retains the hexagonal shape as it grows, even if a change in atmospheric conditions causes a change in the crystal.
The shape of the final snowflake is determined by the exact path it took through the air after it left the cloud. Although each of the six arms of an individual snowflake took the same path, no two snowflakes follow the exact same path. Other environmental factors, such as collisions with other snowflakes and other debris in the air, combined with varying atmospheric conditions account for the great number of possible formations snow crystals can take on, meaning that, ultimately, no two snowflakes are ever exactly alike.
From sledding to snowball fights, ways to enjoy the winter weather are in no short supply. Keep the exploration going with these two kits from hBARSCI:
This 199-piece Water Molecular Lattice Model Kit provides a tangible and hands-on display of the molecular properties of water (H2O). A clean and effective tool for teachers, or a project and study-aid for students, the model requires assembly and attrition that will reinforce the study of water.
This Innovating Science snow polymer kit is used to demonstrate the super-absorbent properties of polymer while making "snow." The kit's hydrophilic polymer instantly absorbs water and expands to over 40 times its original volume, resulting in flakes of fake snow. For hands-on learning, the kit has materials for five demonstrations, and includes, snow polymer, cups, and stir sticks. The kit includes an instructions and safety data sheet (SDS) packet for product information and handling procedures, and is suitable for age 13 and older with adult supervision.