Have you ever looked at a snowflake and wondered how it formed or why it looks different from other snow you might have seen? 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.
What are common snowflake shapes?
Generally, six-sided hexagonal crystals are shaped in high clouds; needles or flat six-sided crystals are shaped in middle height clouds; and 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.
- 32-25° F - Thin hexagonal plates
- 25-21° F - Needles
- 21-14° F - Hollow columns
- 14-10° F - Sector plates (hexagons with indentations)
- 10-3° F - Dendrites (lacy hexagonal shapes)
Why are snowflakes symmetrical (same on all sides)?
First, not all snowflakes are the same on all sides. Uneven temperatures, presence of dirt, and other factors may cause a snowflake to be lop-sided. Yet it is true that many snowflakes are symmetrical and intricate. This is because a snowflake's shape reflects the internal order of the water molecules. Water molecules in the solid state, such as in ice and snow, form weak bonds (called hydrogen bonds) with one another. These ordered arrangements result in the symmetrical, hexagonal shape of the snowflake. During crystallization, the water molecules align themselves to maximize attractive forces and minimize repulsive forces. Consequently, water molecules arrange themselves in predetermined spaces and in a specific arrangement. Water molecules simply arrange themselves to fit the spaces and maintain symmetry.
Is it true that no two snowflakes are identical?
Yes and no. No two snowflakes are exactly identical, down to the precise number of water molecules, spin of electrons, isotope abundance of hydrogen and oxygen, etc. On the other hand, it is possible for two snowflakes to look exactly alike and any given snowflake probably has had a good match at some point in history. Since so many factors affect the structure of a snowflake and since a snowflake's structure is constantly changing in response to environmental conditions, it is improbable that anyone would see two identical snowflakes.
If water and ice are clear, then why does snow look white?
The short answer is that snowflakes have so many light-reflecting surfaces they scatter the light into all of its colors, so snow appears white. The longer answer has to do with the way the human eye perceives color. Even though the light source might not be truly 'white' light (e.g., sunlight, fluorescent, and incandescent all have a particular color), the human brain compensates for a light source. Thus, even though sunlight is yellow and scattered light from snow is yellow, the brain sees snow as white because the whole picture received by the brain has a yellow tint that is automatically subtracted.
It may be hard to find two snowflakes that look identical, but you can classify snow crystals according to their shapes. This is a list of different snowflake patterns.
Hexagonal Plates
Wilson A. BentleyHexagonal plates are six-sides flat shapes. The plates may be simple hexagons or they may be patterned. Sometimes you can see a star pattern in the center of a hexagonal plate.
Stellar Plates
Wilson A. BentleyThese shapes are more common than the simple hexagons. The term 'stellar' is applied to any snowflake shape that radiates outward, like a star. Stellar plates are hexagonal plates that have bumps or simple, unbranched arms.
Stellar Dendrites
Wilson A. BentleyStellar dendrites are a common snowflake shape. These are the branching six-sided shapes most people associate with snowflakes.
Fernlike Stellar Dendrites
Wilson A. BentleyIf the branches extending from a snowflake look feathery or like the fronds of a fern, then the snowflakes are categorized as fernlike stellar dendrites.
Needles
USDA Beltsville Agricultural Research CenterSnow sometimes occurs as fine needles. The needles may be solid, hollow, or partially hollow. Snow crystals tend to form needle shapes when the temperature is around -5°C.
Columns
USDA Beltsville Agricultural Research StationSome snowflakes are six-sided columns. The columns may be short and squat or long and thin. Some columns may be capped. Sometimes (rarely) the columns are twisted. Twisted columns are also called Tsuzumi-shaped snow crystals.
Bullets
USDA Beltsville Agricultural Research CenterColumn-shaped snowflakes sometimes taper at one end, forming a bullet shape. When the bullet-shaped crystals are joined together they can form icy rosettes.
Irregular Shapes
USDA Beltsville Agricultural Research CenterMost snowflakes are imperfect. They may have grown unevenly, broken, melted and refrozen, or had contact with other crystals.
Rimed Crystals
USDA Beltsville Agricultural Research StationSometimes snow crystals come in contact with water vapor from clouds or warmer air. When the water freezes onto the original crystal it forms a coating that is known as rime. Sometimes rime appears as dots or spots on a snowflake. Sometimes rime completely covers the crystal. A crystal coated with rime is called graupel.
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