Forgive my ignorance but I don't see how you can animate a single frame. The gifs you have at the start of this thread are multiple frames compressed into a single image. They could equally have been turned into a mov or mp4 ...
Yeah, sorry, those gifs were not the best examples. It was the best I could dig up fast.
You can use motion vectors to blend between frames like shown in the following video:
Its good because you can get a lot of mileage out of very low framerate material. Especially for realtime graphics.
You can even use motionvectors to blend a single frame and get quite realistic percieved motion from a static image. It will however result in some ghosting artefacts but its usually not too obvious.
This video shows single frame animation of the skies in Starwars Battlefront I. Due to the speed up you can really tell its just one frame but you dont notice it as much at regular playback speeds.
If you had the option to generate motion vectors for clouds in terragen you would be able to cut rendering time for timelapses by a factor of 10. (Render every 10 frame and let the motion vectors blend between them).