Kenneth Libbrecht, professor of physics at Caltech, grows synthetic snowflakes to investigate the physics of crystal growth. His photographs of snowflakes won this year's Lennart Nilsson Award, which is presented annually for outstanding scientific and medical photography.
On his Web site, Libbrecht has a Snow Crystal Primer that explains how snowflakes are born:
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.