by James Morris
Illustrations by Hannah King
What is snow?
We do not know.
But snow is lots of fun,
~P. D. Eastman
What is snow? That’s a simple question. Everyone knows what snow is. But when I asked my teenage son, he wasn’t sure. He of course knew it’s kind of like ice, but when I asked why it doesn’t look like frozen rain (sleet), he wasn’t sure. I then asked several other people, both kids and adults, and they were also unsure.
This is one of those everyday questions that is harder to answer than we think. Snow is something familiar and at the same time unfamiliar.
Let’s start simply. All matter comes in three forms – solid, liquid, and gas. Water is no exception, and its three forms are familiar to us. The solid is ice, the liquid is water, and the gas is water vapor.
Water has many unusual properties. For example, it is “sticky.” One way we recognize this stickiness is surface tension. Water striders, a kind of insect, can walk on water, as their legs don’t break the water’s surface. Similarly, a column of water can be pulled up channels in plants; as water evaporates from leaves, it draws water up from the roots, through the stems, and into the leaves.
This property of water can be explained by its structure. Water is made up of two atoms of hydrogen and one atom of oxygen, written as H2O. The hydrogen atoms carry a slight positive charge, and the oxygen atom has a slight negative charge. Therefore, water molecules interact with one another, with the positive side of one molecule attracting the negative side of another. Individually, each bond is weak, but together, they are strong and account for water’s stickiness.
Hydrogen bonds can also explain another unusual aspect of water. Typically, solids are more dense than liquids. For example, solid silver is denser than liquid silver. This is because the atoms that make up the solid are packed together more closely in the solid compared to the liquid.
But this is not the case with water. For water, it’s just the opposite: liquid water is more dense than frozen ice. As a result, ice forms on the top of water rather than on the bottom. This is significant for life on Earth. It means that lakes and ponds freeze from the top down, not the bottom up, so fish and other aquatic organisms can survive under a roof of ice.
The reason why water is denser than ice has to do with their structures. In ice, each water molecule interacts with exactly four other water molecules in a precise geometrical arrangement. In water, the bonds break and reform, so the molecules are packed together more tightly in water than they are in ice.
Now let’s get back to snow. Snow and conventional ice have the same underlying structure, with a regular spacing of water molecules. But they form differently and therefore take on different outward shapes and end up appearing quite different from each other.
When the temperature goes up, solids turn to liquids that then turn to gases. When the temperature goes down, gases turn to liquids that turn to solids. That’s the “normal” way things work.
But occasionally, under particular circumstances, solids can skip the liquid phase and go directly to the gas phase. And the reverse can happen too. Gases can skip the liquid phase and go right to the solid phase.
This is the first unusual aspect of snow. Snow forms high in the atmosphere in moisture-laden clouds under cold temperatures, below the freezing point of water. The water vapor in the atmosphere (a gas) turns directly into crystals (a solid) that we know as snowflakes. As a kindergartner put it, snow is “frozen clouds,” which is about right. By contrast, the ice we are most familiar with forms when liquid water solidifies.
A single snowflake starts from a small ice crystal that typically forms on its own or around a tiny particle of dust, pollen, or even a bacterium. Once seeded in this way, it grows as it falls, with more water vapor crystallizing around this central point. Snowflakes have six-fold symmetry because ice crystals form six-sided (hexagonal) structures, typically a prism or more complex shapes with branches.
Each snowflake is different because each encounters slightly different conditions (particularly temperature and humidity) as it falls to the ground. These different forms have been beautifully captured and photographed by Vermont native Wilson A. Bentley, whose story is retold in the children’s book, Snowflake Bentley.
So snow and conventional ice have similar molecular structures, but form differently and therefore take on different outward shapes. Other solids can be even more different than ice and snow. Solids of the same material can sometimes be different because of differences in their molecular structures. Carbon is a good example. Solid forms of carbon include diamond and graphite. Both are carbon solids, but have different colors, shapes, and properties.
Ice, you might be surprised to learn, has something like 15 different solid forms that differ in the way that the water molecules are arranged. They go by names like “ice-seven” and “ice-eight.” Kurt Vonnegut even wrote a story, Cat’s Cradle, about a fictitious form of ice called “ice-nine” (different from its real-world counterpart), which has the unusual property of being able to crystallize water that it contacts.
Conventional ice and snow are both “hexagonal ice,” but form differently so don’t look the same. These are the only natural forms of ice on Earth; the others are formed in the lab.
The colors of ice and snow give hints of their structures. Ice is almost clear, whereas snow is white. Because the water molecules in ice are widely spaced, most of the light that hits it is transmitted through. If you look under a microscope at a single snowflake, each part is also clear. But, because there are so many different surfaces in a single snowflake, when light hits a snowflake or a snow bank, it is scattered in all directions, which we perceive as white.
Snow is not that complicated, and yet few of us can confidently describe what it is or how it is different from typical ice, in spite of years of science education. Schooling sometimes does little to help us understand the world around us.
I remember myself not knowing exactly where the liver is or what it does until I went to medical school in spite of years of high school and college biology classes. I now teach college biology, and note that most biology students don’t realize that a sponge is an animal, not a plant.
While not all learning needs to be practical, these everyday experiences can provide reference points and motivation for students. At the very least, education should help students understand, appreciate, and navigate the world around them, even when it’s snowing.
For more information about snowflakes, see this wonderful and informative primer.
© James Morris and Science Whys, 2015.