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Blowin' in the Wind, Issue #070 What types of snow we can see - Sept 1, 2009
September 01, 2009
How do different types of snow form?
You might think we have an infinite number of types of snow. Afterall, everyone has probably heard the expression, "no two snowflakes are alike." But, anyone who has seen snowflakes land on a dark surface (such as one's ski jacket) knows that there are also many different kinds of snowflakes. See http://www.its.caltech.edu/~atomic/snowcrystals/class/class.htm for sketches. There are the classical 6-sided hexagon plates, often with dendritic fingers that give the plate a star-like appearance. And there are columns, needles, bullets, sheaths, cups and an array of mixed variations. The shapes of snowflakes all hinge on the atomic level bonding of hydrogen and water molecules into an ice crystal lattice.
Perhaps the earliest research into snowflakes (including the "no two snowflakes" description above) can be linked to Wilson "Snowflake" Bentley, a Vermont farmer who became fascinated with them. Self-taught, but highly inquisitive, he started photographing snowflakes in 1885, eventually adding a microscopic apparatus to his bellows camera. His pioneering work (and thousands of his snowflake pictures) started us on the road to understanding snowflakes.
More than one snowflake, reallySometimes snowflakes stick together to form clusters of flakes. At other times, the flakes can become rimed, meaning that they accumulate some water either by melting or interaction with atmospheric water droplets. Excessive riming leads to the formation of graupel (also known as soft hail). Sleet is just rain or melted snow that refreezes while falling.
Each of the different types of structures tells something about the temperature (and altitude) at which the flakes formed.
What is snow?Snow forms when atmospheric water vapor transforms immediately to a solid form of water. The process is called sublimation, a vapour-to-solid deposition that completely bypasses the liquid water phase. For the deposition to occur, the vapor must freeze onto something solid floating in the sky. Dust particles, bacteria, pollen, salt spray particles and even solid pollutants can serve the role. These solid particles, know as condensation or deposition nuclei; become the core of any type of precipitation (liquid or solid).
Once the snow crystal forms, it can continue to grow outward by further deposition, as the existing flake serves the role of a deposition nucleus.
Dr. Kenneth G. Libbrecht, a snow researcher at the California Institute of Technology (Caltech) has developed an easy to understand snow morphology (structure or form) diagram. See it at http://www.its.caltech.edu/~atomic/snowcrystals/primer/primer.htm Basically, plates and dendrites form at temperatures just below freezing. As the atmosphere gets colder (from about 25 degrees Fahrenheit to 15 °F), columns dominate. From 15 °F to about -5 °F, plates again dominate. Below -5 °F, columns and plates both form.
The actual type of crystal formation depends on the amount of supersaturation that exists in the clouds. Supersaturation means that the relative humidity in a cloud is actually slightly more than 100%. The more supersaturation, the more likely that dendrites will form.
Libbrecht notes, however, that, the physics behind snow crystal formation and growth is complex and not well understood. So he and others are continuing their research.
But, ski resorts have already launched into artificial snowmaking efforts. Using a snow gun (water source and compressed air used to atomize the water), small snow crystals can be created under the right conditions of temperature, moisture, wind and other variables. Sometimes a nucleator, often a bacterium, is used to assist in the process.
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