In its simplest form, a snow drum is a cylinder with 3/4" plywood ends, 18-24" in diameter, and
hardware cloth (rabbit wire mesh) with holes 1/4-1/2" forming the circumference. A pipe runs horizontally through the center and acts as an axis. Once the drum is filled with shredded polyethylene flakes (
artificial snow), rotating the drum causes the flakes to work through the openings in the mesh and fall to stage.
A much more elaborate and costly unit is manufactured by City Theatrical, and has DMX control.
DMX Snow Machine for Stage, Display and Film Projection
For a full stage effect, a
snow cradle is usually preferred, as long as the venue has a counterweight fly system with two adjacent linesets available. See also the site
http://www.fxtumblers.com/.