For a first (real) attempt, it's pretty darned impressive in my book.
My first comment however, would be that there appears to be a curvature to it, the main clouds dip in the center making the clouds on the left and right edges look closer.
The second comment would be that the directions of the wispier clouds at the top seem to shift around a lot, particularly the ' \ ' angled ones in the very top left which seem to contradict the movement/flow of the other clouds and shading.
I think it mostly boils down to deciding upon which way the clouds are moving, and thus from which direction they are forming.. in the case of the small 'mackerel skies' type clouds in the top right, they seem to (given the general '/' direction of the rest) be more like clouds dissipating, rather than small puffs of cloud growing larger as they travel and grow.
Bottom half is lovely, soft and fluffy looking with a good indication of movement (just needs to be a bit more level horizontally.. especially if you plan on looping it) Top half has a few too many indecisive directions I feel, but that's a lovely blue showing through at the top.
-Z-