Amateur storm chaser and videographer Skip Talbot explains:

The video was shot at 30 fps, and then sped up four times. When it’s sped up, four frames are blended into one. This 4x video was sped up another four times, the frames blended again. The result is very smooth. The trick is starting with video that’s been shot with exposure times close to the length of time between frames. I get this automatically using a camcorder. A still time-lapse with large time gaps between frames would require filters to extend exposure time and eliminate stutter

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