The only way would be ping pong style animation, but those look weird.
Depending on what you need, you can animate phase value and loop footage itself in video editing software of your choice.
It's not true loop cause you blend beginning and end with opacity, and if you need clouds that move a lot it won't work either.
If you're doing timelapse, you maybe can animate amount of clouds. That way you could go from clear to cloudy sky, and back to clear.