We will do this as a 20 second loop, scale this method for other lengths.
Create a 20 second 1 frame comp.
Place your video on layer 1. Nudge the video so that you have some loose frames at the head (also make sure that you have loose frames at the end) Now at the 20 second mark set the out for that layer (alt- close square bracket) Also set the in for that layer at the :0 mark (alt-open square bracket)
Now duplicate that layer. Take th head of the layer and nidge the layer so that the start of that layer is now at 20:01. Now take the in-handle and drag to the left (revealing more in frames) Drag the in-handle to let’s say the 15 second mark. Now set an opacity keyframe (0 transparency) and then go to around 18 seconds and set 100 transparency, this will create a dissolve (note, make sure that this is the uppermost layer).
Now change the duration of the comp toy 20:00 and render out. This new movie will loop as many as times as needed. You shouldn’t see the dissolve too much as clouds do change over time.
Good luck,
Mark