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.
The water molecules in an ice crystal form a hexagonal lattice, as shown at right (the two structures show different views of the same crystal). Each red ball represents an oxygen atom, while the grey sticks represent hydrogen atoms. There are two hydrogens for each oxygen, so the chemical formula is H2 O. The six-fold symmetry of snow crystals ultimately derives from the six-fold symmetry of the ice crystal lattice.
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 simplest snowflakes
When snow crystals are very small, they are mostly in the form of simple hexagonal prisms. But as they grow, branches sprout from the corners to make more complex shapes. Snowflake Branching describes how this happens.
The Morphology Diagram
By growing snow crystals in the laboratory under controlled conditions, one finds that their shapes depend on the temperature and humidity. This behavior is summarized in the "morphology diagram," shown at left, which gives the crystal shape under different conditions. Click on the picture for a closer view.
The morphology diagram tells us a great deal about what kinds of snow crystals form under what conditions. For example, we see that thin plates and stars grow around -2 C (28 F), while columns and slender needles appear near -5 C (23 F). Plates and stars again form near -15 C (5 F), and a combination of plates and columns are made around -30 C (-22 F).
Why snow crystal shapes change so much with temperature remains something of a scientific mystery.
The growth depends on exactly how water vapor molecules are incorporated into the growing ice crystal, and the physics behind this is complex and not well understood. It is the subject of current research in my lab and elsewhere.
The life of a snowflake
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.
The rest of the story