Thanks to each of you for your generous comments.
Walt, that is a keen observation. But no, it's not due to a gradient. I went back and checked, and the tone difference in the upper part of the mountains was not introduced by post processing. I think it is probably a combination of two things. First, despite the smoke-clearing effect of the incoming storm, a slight haze still existed closer to the ground, which tended to reduce contrast and brighten that area. Second, the lighting on the top of the peaks are most affected by the dark clouds right above them, whereas the lower areas are receiving a little more light from the less-cloudy sky between them and the camera.
