>>507917376 (OP)as rockets get higher up, the atmosphere gets thinner, and so the high pressure plume spreads out more.
when it happens in the early morning or evening (close to sunrise/sunset), it is high enough for it to be dark on the ground but still in sunlight at altitude, giving it a spectacular glowing effect as viewed from the ground
https://en.wikipedia.org/wiki/Space_jellyfish
https://en.wikipedia.org/wiki/Twilight_phenomenon
https://youtu.be/Y1Hfiirwgys