Snow Crystals
“It’s snowing!” has been a familiar refrain over the holidays and into the New Year. Love it or hate it, we usually experience at least one snowfall every winter. Taking a closer look at snow, and the individual beauty of snowflakes, may give us an all new appreciation of the dreaded (to some) white stuff. The question of how snowflakes are created is a complex subject that involves a lot of physics. But this shouldn’t stop us from trying to understand a few basics.
First of all, a snow crystal is simply a single crystal of ice. A snowflake may be made up of one snow crystal, or several that have clumped together. Snow crystals are formed in the clouds, when water vapor condenses into ice. Raindrops that freeze as they fall are called sleet, and bear no resemblance to the intricately patterned snow crystals.
Water vapor in the air comes largely from the evaporation of oceans, lakes and rivers. When air holding water vapor is cooled to a certain point, the water vapor begins to condense out. Near ground, it may condense into dew on the grass. In the sky, water vapor condenses around particles of dust. Clouds are made up of millions of tiny water droplets, each of which contains one or more dust particles.
Snow clouds are made up mostly of water droplets. The droplets may remain liquid even when the temperature dips below freezing. The droplets at this point are “supercooled”. At around -10º C droplets gradually begin freezing.
When a droplet of water freezes, it becomes a small particle of ice. As water vapor condenses onto its surface, the ice grows, creating a snow crystal. When the snow crystal gets larger, remaining water droplets begin to evaporate, returning water vapor to the cloud. As snow falls there is a complex flow of water between the liquid state (water droplets) and the solid state (ice crystals).
Each ice crystal is made up of oxygen and hydrogen (H20) molecules that are arranged in a hexagonal lattice. In a liquid state, water molecules move around freely, but in a solid state, they become arranged in the six-sided symmetry of the lattice.
A hexagonal prism, which may be plate-like or columnar, is the most basic form of snow crystal. As the snow crystal grows, branches sprout from the six corners to create complex and elaborate designs. The shape and complexity of each snowflake depends on factors of temperature and humidity. For example, thin plates and stars grow around -2º C, while columns and slender needles grow around -5º C. Despite being well studied by scientists, the reason why snow crystals change so much with temperature is something of a mystery.
Like many things in nature, the design of each snowflake is a spontaneous and random process. Contrary to the way humans do things, there are no blueprints or plans. Snowflakes literally come out of thin air, and each snowflake is unique because no two snowflakes fall in exactly the same way, through the same conditions, in their descent from the clouds.
The discovery that no two snowflakes are exactly alike was made by Wilson A. Bentley (1865-1931), a Vermont farmer who avidly studied and photographed snowflakes. Wilson, who went by the nickname “Snowflake” was the first person to photograph a single snow crystal in 1885, using a microscope and a bellows camera. During his lifetime he photographed more than 5000 snowflakes.
In Bentley’s words “Under the microscope, I found that snowflakes were miracles of beauty; and it seemed a shame that this beauty should not be seen and appreciated by others. Every crystal was a masterpiece of design and no one design was every repeated. When a snowflake melted, that design was forever lost. Just that much beauty was gone, without leaving any record behind.”
Snowflake watching is a fun activity for a winter’s day. Though some larger snowflakes can be appreciated with a naked eye, an inexpensive hand-lens or magnifying glass will give one a much better look. The many beautiful shapes of snowflakes, including plates, columns, dendrites and more are amazing to look at.
Marveling at the pattern of a snowflake, and the complex and haphazard process of its creation may give us a different perspective on snow. Even in the depths of winter, it is nice to know that there is something beautiful and interesting to look at right outside the door.
Kenneth Libbrecht, a physicist who studies snowflakes, has written several books on the subject, including a field guide to snowflake-watching. Libbrecht’s web site, http://www.snowcrystals.com, is a great source of information about snow crystals, with galleries of stunning photographs.
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