I threw this together really quick to explain. Black is the actual screen area, the grey is what the player wouldn't be able to see.



The trail explains the movement I was implying with your cloud. It invokes a more natural feeling than just instant up and instant down.

You'll also note the enemy clouds on the left. In the same way you create clouds outside the boundaries of the screen on the right, you'll want to destroy them outside of the boundaries on the left.