Actually it's not a "trick" but a photographic technique, related to multiple exposure.
It's called "cloud stacking". Done on the top of Wellington Cable Car.
This particular image is composed of 64 individual shots of the same scene, with a gap of 15 seconds between shots. Those images are later combined in post processing (50 years ago this would have been done in a darkroom). Usually this is done with daylight shots, close to sunrise/set, as the colors shift and give the impression of a painting in the sky. As I am more of a night creature, I thought to try it in a moonlit windy night, getting those "tractor tracks".
Back when I first posted it, It was also shared on the Cable Car's website.
The second image shows one of those individual shots.
Prints are of course available (of both versions), feel free to contact me if you're interested!