Now that's a nice one. It's a volumetric rendering sequence of a... well, not a smokey tree which grows as a timelapse into the sky ;) - no, it's a nuclear explosion cloud rendering. This Explosion yields between 5 to 8 kilotons (less than the hiroshima bomb) and would be produced by a "tactical nuclear warhead" with a relatively low yield.
It had a somehow complex scene setup and took ages to render (okay... 11 hours with a 1080p resolution). The Alpha is added at the end of the sequence and has a really nice volumetric flow. I use it myself for my own short movie projects or for a "just for fun" composite project.

There is a fully composited Scene with this Explosion, right here:
Nuclear Explosion Scene Composite