I was doing some more WebGL work and thought it would be interesting to do a tornado effect, so I came up with this.
I believe the effect here closely mimics how things were done in Sacrifice and Giants: Citizen Kabuto. I'm using a Bezier curve to construct and animate the funnel. I didn't have any decent textures on hand to emit a particle cloud around the funnel (which would give it a nice puffy look), so you could say it's a naked twister
I used 4 control points, which allows the tornado to bend in interesting ways. The screenshot below is an example I created in Blender. This curve has only 3 control points.
You can use the middle and ground points to control the twist and curve of the tornado, simulating ground wind.
You calculate the mesh by using a tubing algorithm (which primarily uses Reed's circular formula). The tubing algorithm is exactly how you would construct a cylinder. You construct multiple circles at each level in the curve. You can add as many levels as you want for more resolution, and each circle can have any number of points. Using the direction vector of the curve, you construct a circle around that point and then connect its edges with the next circle you will create in the curve. Each control point contains a radius, which defines the size of the tube at that point. You linearly interpolate that radius as you move along the curve. So in the example above, the ground point has a radius of 2, the middle has a radius of 5, and the top has a radius of 10.
The wireframe should look something like this.
Now all you have to do is write logic to move the tornado around. Due to the nature of Bezier curves and how the 3D meshes are constructed, you can also animate the birth of a tornado. You can display the top portion of the funnel and slowly build on it until it connects with the ground. Freebie effect.
On another note, this same algorithm can be used to generate trees.