http://freeweb.pdq.net/headstrong/hurr.htm

Listed below are the materials we used:

         4 pieces plywood 3/4" x 4 1/2" x 16" (solid boards used)
             (19mm x 114mm x 406mm) 
             4 pieces plywood 3/4" x 4 1/2" x 14 1/2" (solid boards used)
             (19mm x 114 mm x 368mm) 
             2 pieces plywood 3/8" x 16" x 16" (paneling used)
             (9.5mm x 406mm x 406mm) 
             4 ceramic standard incandescent light bulb holders 
             wiring for above 
             3 pieces glass 14" x 20" (355mm x 508mm) (lexan used)
             1 piece masonite 14" x 20" (cardboard used)
             quarter round, 21 feet (6.5m) (rectangular strips used)
             four 2-foot (610 mm) sections of 1" (25mm) angle iron (not used)
             pie pan (coffee can used)
             stove or vent pipe  (coffee cans used, with a fan because of the dry ice)
             hot plate (dry ice used) 

For safety reason dry ice was used instead of a hot plate and boiling water.  While the original water and steam more closely mimics tornadoes in nature, the cool mist from the dry ice required a fan to cause an updraft that occurs naturally when steam is used. 

The plans we used for the Tornado Machine are included as Appendix A.....Page 8, or through the link, above.

As the air, marked by the mist, rises, it is turned in a circle by the air coming in through the slots on side of the machine.  This "twister" looks and acts like a tornado.