The crystals shown above were all grown as they fell slowly through a large growth chamber, which is shown schematically at right. The insulated copper tank was chilled to a constant temperature using a computer-controlled refrigerator, and water vapor was supplied by two containers of warm water. Since the water was at the bottom of the chamber, there was substantial convective mixing, which produced supersaturated air with only fairly small temperature variations. To nucleate snow crystals, a tiny flake of dry ice was dropped into the tank from the top, typically producing hundreds of minute crystallites via homogeneous nucleation. When the crystals are very small they fall very slowly, and thus tend to precipitate out of the chamber only after they've grown to a size of about 10 - 100 microns.
Some crystals fall onto the observing window, where they can be photographed.
Thus we see that most of the crystals photographed above were smaller than the diameter of a human hair. At the far right we have a picture of the growth chamber, which is covered with styrofoam insulation and topped with a high-intensity lamp. The refrigerator is at the lower left in the image.