Very nice. Here's my inexpert opinion: I think this one is a good example that the "thirds" rule needs exceptions. I don't think any of the sky or the foreground rocks should be cropped out. The sky looks like it's probably more uniform above the frame, at least that's the trend it seems to be exhibiting as your eye moves up, so I don't know what would've been gained by aiming up a little more, and going the other way would've excluded a lot of what gives it the ominous tone. And anyway, as I write this I realize that it does possess the thirds thing. I amend my previous statement somewhat (although I still think that trying to excessively and rigidly adhere to any compositional rule is counterproductive): In a shot that includes land or water and sky, the horizon doesn't have to be the horizontal demarcation of thirds. In this case, for me anyway, it's the transition zones from brighter sky to the dark, ominous clouds.
Just the ramblings of an extreme noob, here. Any pretentiousness is accidental (I just like to hear myself talk).