Above: A hexagonal (six-sided) snow crystal, classified as a stellar dendrite. This crystal was photographed with an Olympus TG-6 camera after it landed on the sleeve of the authors fleece jacket. (Tom Niziol)
They are one of the most beautiful offerings of nature–otherworldly, the topic of scientific scrutiny, and in some parts of the world, almost a daily winter occurrence. Yet, most people have not taken the time to actually look closely at those snowflakes or snow crystals that fall from the sky. If they did, here is what they might see.
As a kid living in Buffalo, New York, I had plenty of time to look at snow. I learned to appreciate snowflakes, or snow crystals as the individual specimens are called, by looking at them when they fell on the coat of our black Labrador retriever. Because his coat insulated him from losing much body heat, those little crystals would stand out beautifully against the dark background. I also found it fascinating that all of these little snow crystals had a hexagonal (six-sided) shape. Later on, I developed an interest in snow photography and have since photographed thousands of snowflakes.
What exactly are snow crystals, how do they form, and why in the world do they have a hexagonal shape? For answers to those questions, we need to first understand the qualities of another miracle of nature—the various forms, or states, that water can take on our planet.
As we learned in school, water has a molecular structure of two hydrogen atoms to every oxygen atom (H2O). In their liquid state, those H2O molecules are spaced far enough apart to “jiggle” around or slide a lot, as noted physicist Robert Feynman would say. They are not held fast, which is what gives water its liquid property.
As the temperature drops, however, those molecules grow closer and closer together and eventually lock into what is referred to as a crystalline array. In the case of water molecules, that array takes on a 3-D hexagonal configuration, as shown below. The hexagonal component emerges because that type of structure is the most efficient for water molecules to “hook” together.
Depending on the thickness of the 3-D hexagonal prism, it can take several forms: a flat plate; the classic branched-crystal form of a dendritic snowflake, or dendrite; a thick hexagonal column not unlike a pencil; or an even stranger shape, as shown below.
In understanding snow crystals, it is important to note that they are not frozen raindrops. In the United States, we would refer to frozen raindrops as sleet, a type of ice pellet. Interestingly, in the United Kingdom, “sleet” refers to mixed rain and snow.
Why snowflakes grow at the expense of water droplets
Snow crystal formation skips the raindrop phase altogether. Instead, water in the form of gas (water vapor) is deposited directly onto an ice nucleus to begin the snow crystal building process. This diffusion deposition occurs because there is a difference in something called “saturation vapor pressure” between ice and liquid water.
The key to understanding this concept is the term “pressure”. Objects tend to move from higher to lower pressure. At a given temperature, the vapor pressure over a water surface is greater than that over an ice surface. So, the water molecules in water vapor (the gaseous form of water) move toward and are deposited onto the ice nucleus. The ice crystals grow at the water molecules’ expense.
Eventually, the water molecules disappear as the ice crystal grows like the one below. If the ice nuclei grow outward in the hexagonal prism, you end up with a flat plates or dendrite with legs growing outward from one of the six points in the hexagonal plate, which “stick out” into the moist air just a bit more than the sides. If the crystal grows upward from the prism face, you get a hexagonal column.
By the way, for you weather geeks out there, that difference in saturation pressure between water and ice is greatest between about –12°C and –18°C (10°F and 0°F). Many of you will be familiar with this temperature range as the dendritic growth zone (DGZ), the most favorable in-cloud temperatures for snow crystal growth. It’s been calculated that it can take as many as one million cloud droplets to provide enough water vapor for a large snow crystal.
There are two other ways for crystals to grow once they form: accretion and aggregation.
Accretion: In this process the ice crystals will continue to grow by collision with supercooled droplets. Water droplets like the tiny ones you find in a cloud will stay in a supercooled liquid state down to about –12°C. When the newly formed snow crystal begins its journey down to the ground through a layer of these supercooled water droplets, they freeze on contact with the snow crystal to form rime ice, as shown in the figure below. Excessive riming will even result in the formation of graupel or snow pellets.
Aggregation: Finally, if the atmosphere is closer to the freezing mark and the humidity is very high, individual snow crystals will begin to stick together as they fall to earth. Their interlocking legs can form huge silver-dollar-sized snowflakes that are sometimes more than 3 centimeters (1.2 inches) in diameter. The “stickiness” of the snow crystals is maximized at or near 0°C (32°F).
No identical snowflakes: truth or myth?
So, what about the statement that “no two snow crystals are exactly the same”? Well, think about the path a snow crystal takes as it falls from way up in the clouds. Every snow crystal falls through a very complex combination of temperature, moisture and pressure conditions. Now, if those crystals are falling close together, they encounter the same general conditions as they fall to earth, i.e. an environment with a certain range of temperature and moisture conditions. However, every crystal takes just a little different path on the way down, as they are each whirled around by tiny wind currents. So, each crystal has its own “designer atmosphere” that ensures it is unique when it hits the earth.
It’s possible for two snow crystals to be almost identical, though. Recently, researchers have been able to actually grow snow crystals in the laboratory, using sophisticated equipment to fine-tune exact conditions of temperature and moisture instead of relying on the haphazard combination of conditions that a snow crystal might undergo as it falls to earth.
I consider Dr. Kenneth Libbrecht, professor of physics at the California University of Technology (Caltech), to be the absolute expert on all things snow crystals. At his website snowcrystals.com, Dr. Libbrecht covers everything you would ever like to learn about the topic and has some of the most beautiful snow crystal photos ever taken.
Under prescribed laboratory conditions, Dr. Libbrecht can not only make his own snow crystals but also design them to his specifications—kind of like a “snowflake god”. In fact, he has been able to make “snowflake twins”, as he calls them, as shown below. Those snowflakes grew next to one another in the controlled laboratory setting. Because they experienced the same changing growth conditions, they grew into nearly the same shape. This can even happen in nature once in a rare while. Back in the 1980s, researcher Nancy Knight at the National Center for Atmospheric Research discovered a set of virtually identical snow crystals that formed in a natural setting.
As much as the snow crystals made in the laboratory setting are great for studying their properties and formation, I still like the random beauty that is created by each and every one of them that falls through Mother Nature’s natural laboratory that we know as our atmosphere. I for one like to believe that each and every snow crystal that falls form the sky has its own character or personality, not unlike every person on the face of the earth. Yes, there are identical twins walking around, but when you get down to the molecular level, you will find differences, just like those snow crystals. I also like the fact that even when a snow crystal may look a bit damaged or imperfect, closer inspection still reveals an absolutely beautiful creation of nature—not unlike every single one of us, who in spite of not being perfect specimens, are still amazing creations of nature. I guess people and snow crystals do have a lot in common.
So, when it begins to snow outside, take the time out to look closely, but don’t breathe on those crystals because they will disappear in an instant. If you have a magnifying glass or a low-cost clip-on closeup lens for your cell phone, even better. You can even take it as far as a microscope and more high-powered camera. You will find more tips on photographing snow crystals at my Facebook page.
Explore, and enjoy what Mother Nature has provided to you for free of charge. Don’t hesitate to take that photo, because chances are very good that you will have captured a unique part of nature that nobody else can copy!
