Back in December, researchers at Caltech posted a research paper to arXiv that attempts to explain why the shape and structure of snowflakes change significantly depending on relatively small shifts in temperature.
In order to study this, they had to grow snowflakes in laboratory conditions. It was not an easy thing to figure out how to do. On his Snowcrystals page, physicist Kenneth G. Libbrecht show you how it's done.
There are many ways to grow snowflakes, but my favorite starts with something called a vapor diffusion chamber. This is essentially nothing more than an insulated box that is kept cold on the bottom (say -40C) and hot on the top (say +40C). A source of water is placed at the top, and water vapor diffuses down through the box, producing supersaturated air. The cold, supersatured air at the center of the chamber is ideal for growing ice crystals.
While working with this diffusion chamber, we rediscovered a wonderful technique for growing synthetic snow crystals that was first published in 1963 by meteorologist Basil Mason and collaborators . One starts by putting a wire into the diffusion chamber from below, so that small ice crystals begin growing on the wire's tip. Then apply a high voltage to the wire, say +2000 volts, and voila — slender ice needles begin growing from the wire.