The solid state of water, ice, is an example of a crystalline solid—a solid that is made from particles (atoms, molecules, or ions) that are arranged in a regular, repeating pattern. The orderly structure of a crystal is due to repeating three-dimensional geometric units that maximize the attractive forces between the atoms while minimizing repulsive forces.

Most naturally occurring ice has a hexagonal structure. Each molecule of water is made of two hydrogen atoms covalently bonded to one oxygen atom. In addition, water molecules also have a weak attraction to one another. As a result of both the covalent bonding and the attraction between molecules, water molecules are most stable as a solid when arranged in layers of hexagonal rings. This layout, known as a hexagonal lattice, ultimately determines the six-sided symmetry of snowflakes.

Crystals form through a process called nucleation, where a solid surface or a group of molecules acts as a seed for the crystal. As more and more molecules bond to this nucleation site, the crystal grows. Snowflakes begin to form when water vapor condenses onto dust particles in the atmosphere; if the temperature is cold enough, the water droplets freeze and act as nucleation sites. As more and more water vapor condenses onto the ice crystal, the snowflake grows.

Water molecules attach more readily to certain parts of an ice crystal as it grows, resulting in the formation of flat surfaces, or facets. The most basic snowflake shape is a hexagonal prism. The intricate designs of more complex snowflakes form as a result of "branching." In this process, water molecules bond to the growing crystal as soon as they reach an appropriate surface, so if there is a protruding bump on the crystal—such as the corner of a basic hexagonal prism—the molecules attach to this bump rather than to other parts of the crystal. As a result, bumps can grow quickly and develop into branches, which, in turn, can develop side-branches, and so on.

It is known that the shape and design of an ice crystal depends on the temperature and humidity at which it forms. However, the reason why snowflakes experience such extreme variations in shape is not well understood. Scientists believe that, since ice is very sensitive to growing conditions, slight variations in atmospheric conditions change the way the crystal grows at any given moment. However, a snowflake still ends up being nearly symmetrical because each part of the snowflake experiences the same changes in conditions at the same time. Amazingly, because every snowflake travels a different path through the atmosphere, and thus experiences unique conditions during its growth, the old saying is most likely true: no two snowflakes are exactly alike.

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