Editing Selected Areas- Part II
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Although this seems like a perfect candidate for the Stack Images tool, in this case it was not. The reason is that another, darker exposure was not made right away. The clouds changed position between exposures, which could be taken care of with Amount Masks, but worse, the two images are not framed exactly the same way. Here is the dark exposure . . .

masking step 1

Although they look identical in these small-scale views, upon close inspection, the dark image is slightly higher and shifted to the right compared to the light image. Since the Stack Images tool does not allow shifting images, the Composite transformation is the way to put the darker sky & clouds with the lighter trees & mountains.