Wednesday, January 25, 2012

What is a Snowflake?

A flake of snow is a feathery ice crystal, typically displaying delicate sixfold symmetry. Snowflakes and snow crystals are made of ice, and pretty much nothing more. A snow crystal is a single crystal of ice. A snowflake can mean an individual snow crystal, or a few snow crystals stuck together, or large agglomerations of snow crystals.



Snowflakes are not frozen raindrops. Sometimes raindrops do freeze as they fall, but this is called sleet. Sleet particles don't have any of the elaborate and symmetrical patterning found in snow crystals. Snow crystals form when water vapor condenses directly into ice, which happens in the clouds. The patterns emerge as the crystals grow.

Have you ever looked at a snowflake and wondered how it formed? Snowflakes are a particular form of water ice. Snowflakes form in clouds, which consist of water vapor. When the temperature is 32° F (0° C) or colder, water changes from its liquid form into ice. Several factors affect snowflake formation. Temperature, air currents, and humidity all influence shape and size. Dirt and dust particles can get mixed up in the water and affect crystal weight and durability. The dirt particles make the snowflake heavier, and can cause cracks and breaks in the crystal and make it easier to melt. Snowflake formation is a dynamic process. A snowflake may encounter many different environmental conditions, sometimes melting it, sometimes causing growth, always changing its structure.

In more detail, here is the story of a snowflake. Each flake of snow begins with water vapor in the air. Evaporation from oceans, lakes, rivers, transpiration from plants, and exhaling from animals. Even you, every time you exhale, put water vapor into the air.   When a mass of air is cooled, the water vapor it holds begins to condense. Right at the surface of the earth, water vapor condenses onto surfaces, such as dew on grass. In the atmosphere, water vapor condenses onto dust particles. A cloud is nothing more than a huge collection of water droplets hugging dust particles, all suspended above the earth.  As clouds gets colder, the droplets in the clouds start to freeze into minute ice crystals.   Liquid water vapor in the cloud then attaches to the ice crystal embryo in a continuous process causing the snowflake to grow.  
Common snowflake shapes include:
  • Generally, six-sided hexagonal crystals are shaped in high clouds.
  • Needles or flat six-sided crystals are shaped in middle height cloud. 
  • A wide variety of six-sided shapes are formed in low clouds.  
  • Colder temperatures produce snowflakes with sharper tips on the sides of the crystals and may lead to branching of the snowflake arms (dendrites).  
  • Snowflakes that grow under warmer conditions grow more slowly, resulting in smoother, less intricate shapes.

Why are snowflakes symmetrical (same on all sides)?  Actually, not all snowflakes are symmetrical. Several things can distort the shape of a snowflake including uneven temperatures, dirt, and other factors But in most cases snowflakes form in an orderly and symmetrical manner. This is because when water freezes, the shape of the ice crystal reflects the internal order of the water molecules. When molecules freeze into the solid state, the bonds or connections between the water molecules are very weak. During crystallization, the water molecules align themselves to maximize these bonds as much as possible. Consequently, water molecules arrange themselves in a chemically predetermined manner that tends to maintain symmetry.

When we were kids we always heard that no two snowflakes are the same. This is not necessarily true. This may be true if you were able to examine each snowflake down to the exact number of water molecules, arrangement of electrons, etc. However, on the macro scale it is possible for two snowflakes to look alike. Over the history of the earth, it is also likely that duplicate snowflakes have been produced.

If water and ice are clear, then why does snow look white?  The main reason is that snowflakes have light-reflecting surfaces that scatter light into its component colors, so snow appears white. Another contributing factor is that there are tendencies in the way that that human eyesight works. The human brain compensates for a light source. Thus, even though sunlight is yellow and scattered light from snow is yellow, the brain automatically subtracts the yellow wavelengths causing you to see white.

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