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- Add Layer Mask to landscape layer.
- In order to smooth layer edges fill horizon area of the Mask with linear black and white gradient 80% Opacity.
- Paste sky image in the bottom layer.
The sky after and before changes.
- To create dramatic and dark effect, adjust levels, just like in the screen shot below, play with Contrast and Color Balance adjustments. Image > Adjustments>
- > Levels
- > Color Balance
- > Hue/Saturation
- > ... and others
- Change Saturation settings.