This looks like a pretty cool toy to build. At the top of the cylinder is a fan that removes air from the center column of the device creating a low pressure zone. This air is then blown back into the cylinder to feed the rotation. Besides the regular construction supplies you’ll need a squirrel cage fan and speed control. The most expensive part of this project is the mist generator. The nice thing is, you can construct the entire device and make sure it’s functioning before you dump money into the mister. I saw a really large one of these last year at NCAR in Boulder, CO. It was pretty cool, but naturally I was there to see the Cray-1A [via]. Lately we’ve been creating our own path of destruction so we’re hosting the pdf which was originally found at weather-photography.com.