Tornadoes (Twisters) are among the most violent and destructive storms. These rotating clouds have a diameter that rarely exceeds 1/4 mile, but have wind speeds on the order of several hundred miles per hour. More tornadoes occur in the U.S.A. than in all other countries combined. Tornadoes have occurred in every state in the contiguous 48 states and have occurred every month of the year.
Most tornadoes occur in conjunction with severe thunderstorms. One of the prerequisites for the formation of severe thunderstorms is instability of the lower atmospheric layers. Another prerequisite appears to be that the air between the ground and the cloud, the subcloud layer, must have some rotation. By imitating natural conditions, you can make a safe working model of a miniature tornado.
The warm moist air in the subcloud layer can be achieved with a simple cake pan which contains heated water. An strong immersion heater works best, but the pan can be heated from the bottom using a hotplate or even by canned heat such as Sterno. The instability and severe thunderstorm can be simulated using a stove pipe chimney through which warm, moist air rises. The rotation can be simulated by forcing the air entering the box through gaps arranged so the air comes in near the corners of a box.
The moisture source is the hot water in a square large baking pan 8" x 8" and 2" deep. The box which models the subcloud layer is made of external grade plywood or particle board as in the diagram. The back two boards are solid and are attached so there is a 2" slot on one side which helps give spin to the air. The front two faces are made of glass or acrylic plastic and also have open 2" slots on one side to admit room air. A board across the top with a hole just smaller than the stovepipe completes the structure. Paint the inside surfaces flat black or line with black velvet for the best effect.
Lighting the tornado is perhaps the most important part of the demonstration as shining a simple light in the window is not very effective. A slide projector shining in one window and viewing through the other gives the best results, which are easily captured on video. Usually, you can see the central clearing where the air is descending in real tornadoes.
Materials: One large baking pan at least 8" x 8" x 2"
Plywood for the structure
Two sheets of clear plastic
Flat black paint or black velvet
Heat source such as an immersion heater or hotplate
Light source such as a slide projector