I was just trying to figure out why you had ISO 640, if your shutter was 1/1600 and aperture f/13? I always try to find the most reasonable combinations that gives me the lowest ISO possible, to avoid noise, even if it's just digital noise. The 7D's sensor is noisy, unfortunately and the 18mp crammed into that sensor causes problems. Why don't you shoot the clouds at f/8, and shutter 1/800, for instance, and keep ISO at 125? (approximate settings).