The life of a snowflake

The life of a snowflake
The diverse morphologies seen in snow crystals are largely due to the bizarre temperature dependence of ice crystal growth rates, a phenomenon that was discovered 75 years ago and remains unexplained to this day. 

  Snowflakes and snow crystals are made of ice, and pretty much nothing more.  A snow crystal, as the name implies, is a single crystal of ice.  A snowflake is a more general term; it can mean an individual snow crystal, or a few snow crystals stuck together, or large agglomerations of snow crystals that form "puff-balls" that float down from the clouds.

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.

The most basic form of a snow crystal is a hexagonal prism. This structure occurs because certain surfaces of the crystal, the facet surfaces, accumulate material very slowly.

The story of a snowflake begins with water vapor in the air.  Evaporation from oceans, lakes, and rivers puts water vapor into the air, as does transpiration from plants.  Even you, every time you exhale, put water vapor into the air.  

   When you take a parcel of air and cool it down, at some point the water vapor it holds will begin to condense out.  When this happens near the ground, the water may condense as dew on the grass.  High above the ground, water vapor condenses onto dust particles in the air.  It condenses into countless minute droplets, where each droplet contains at least one dust particle.  A cloud is nothing more than a huge collection of these water droplets suspended in the air.

   In the winter, snow forming clouds are still mostly made of liquid water droplets, even when the temperature is below freezing.  The water is said to be supercooled, meaning simply that it is cooled below the freezing point.  As the clouds gets colder, however, the droplets do start to freeze.  This begins happening around -10 C (14 F), but it's a gradual process and the droplets don't all freeze at once.
 
   If a particular droplet freezes, it becomes a small particle of ice surrounded by the remaining liquid water droplets in the cloud.  The ice grows as water vapor condenses onto its surface, forming a snowflake in the process.  As the ice grows larger, the remaining water droplets slowly evaporate and put more water vapor into the air.  

   Note what happens to the water,  it evaporates from the water droplets and goes into the air, and it comes out of the air as it condenses on the growing snow crystals.  As the snow falls there is a net flow of water from the liquid state (cloud droplets) to the solid state (snowflakes).  This rather complicated chain of events is how a cloud freezes. 

Source and further reading:
http://www.its.caltech.edu/~atomic/publist/rpp5_4_R03.pdf