Crystals

 
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Source: Marcia Politovich, National Center for Atmospheric Research

Crystals

Temperature and humidity shape snow crystals.  As snow crystals form they take on a six-sided or hexagonal shape but with what seems like an infinite number of variations of being six-sided.  Snow crystals form hexagonal shapes because of the way the two hydrogen atoms that join with an oxygen atom to form a water molecule (H₂O) attach to the hydrogen atoms of other water molecules.

The temperature at which a crystal forms and to less extent the humidity of the air, determine the basic shape.  The many things that happen to snow crystals as they fall, collisions, partial melting, and colliding with water drops that freeze to them create even more shapes. 

This is why irregular crystals with no easily identifiable form are the most common.  Some times crystals are a combination of more than one form.  For example, hollow columns that form in air colder than -8 Fahrenheit could grow thin plates on one or both ends as they fall through warmer air. 

While most people refer to shapes like those in the graphic above as snowflakes, flakes are really made of many snow crystals that have stuck together.

Some common snow crystals:

	1a Columns	Growth at high supersaturation at -3º to -8ºC and below -22ºC	Short prismatic crystal, solid or hollow
	1b Needles	Growth at high supersaturation at -3º to -5ºC	Needle-like, approximately cylindrical
	1c Plates	Growth at high supersaturation at 0º to -3ºC and -8º to -25ºC	Plate-like, mostly hexagonal
	1d Stellar Crystals	Growth at high supersaturation at temperatures between -12º to -16ºC	Six-fold star-like, planar or spatial

The International Commission on Snow and Ice (ICSI) also has a category for irregular crystals and classes for hail and ice pellets.

If you are interested in learning more about snow crystals follow these links:

• Wilson "Snowflake" Bentley
  

• Kenneth G. Libbrecht's "Snow Crystals"
  

• Scientific America's, "Why do Snowflakes Crystallize...?
  

Sources for the thumbnails:

Columns--Two Japanese Researchers, Kobayashi & Kuroda
Needles--One of the very first avalanche forecasters, Edward LaChepelle
Plate & Stellar--The very first person to photograph a snow crystal, Wilson A. Bentley