there's no motion blurr in bryce so more then 180 degrees rotation per frame creates an effect that looks like the tornado is spinning backwards and at more then 360 degrees rotation it starts looking like it's rotating forwards again and so forth. Your tornado looks like it's spinning backwards with 99 degrees per frame. You could change the texture of your tornado to make it look like it's already motionblurred and then animate the texture and rotate your model slightly to create the larger movements. flying debris will also make the effect more dramatic. If you look at real tornado footage(sp?), you don't see the tornado spinning almost 41.625 times per second.

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