Why do snowflakes form unique shapes?

[nst.john]

I'm not sure if this is the right forum to put it in if not please move it. But anyway I wanted to ask why snow forms flakes instead of like frozen rain drops when it snows. On a molecular or atomic level what is going on with the atoms and water molecules to turn frozen water into snowflakes.

 

[eigenperson]

Snow doesn't come in the form of frozen raindrops because it was never rain in the first place.

During the winter in temperate climates, there are no raindrops in the clouds. Instead, a cloud contains a mixture of supercooled water droplets and ice crystals. The droplets evaporate more quickly than the crystals sublime, so over time water vapor evaporates from the droplets and is deposited onto the ice crystals. This causes the crystals to grow. The exact shape of the crystal depends on the temperature and humidity in which the growth occurs, but in general respects the hexagonal crystal structure of ice. The crystals can also stick together or stick to water droplets (which freeze when they contact the crystals). Eventually they become too heavy to be supported by the updrafts in the cloud and fall as snowflakes.

If the temperature near the ground is above freezing, the snow will then melt to form rain. Of course, once a snowflake melts, it loses its crystalline shape and becomes just a blob.

Sometimes frozen raindrops do fall (called "sleet" in the US and Canada and "ice pellets" elsewhere). This happens when snow melts into rain on the way down and then refreezes in a cold layer near the ground. This results in frozen blobs shaped roughly like raindrops, just as you would expect.

 

[nst.john]

Huh. That's really interesting. Thanks a lot for the good answer.

 

[bahamagreen]

The exact shape of the crystal depends on the temperature and humidity in which the growth occurs...

This is the part responsible for the idea that all snowflakes are different. The formation is occurring at the micro level, and the conditions of temperature, humidity, and a few others are also varying at the microlevel as the flake makes its path through the air.

These microlevel variations in the conditions of the air, and the building of the flake structure influenced by these microlevel variables along its path through the air, and the sense that each individual flake takes a unique path through the air to the ground... all results in a unique history of construction of each the flake, each moment of which is in response to the very micro-local conditions, and which history of conditions (the particular sequence of these variations) will be different for each flake path to the ground.

 

[Maui]

It's because of a process called symmetry breaking and technically it's not exactly known why snowflakes form as they do but it's probably related to the molecular structure of water: