Bardill.net: Replace the boring sky

Replace the Boring Sky

Isolate the sky and replace it with natural blue.

Original photo.

The sky has been replaced with a gradient.

In the Channels window, click on the Red, Green, and Blue Channels to determine which Channel has the greatest contrast between the sky/fog and non-sky elements.
Right-click on the chosen Channel, and choose Duplicate. You might rename it to "Sky Selection" or whatever suits you.
Enhance the contrast in your sky selection Channel by using Curves. Select "Image>Adjust>Curves".
Select "Image>Adjust>Threshold". Slide the threshold up or down to isolate the sky as much as possible.
You will note that the effect is imperfect. It is usually necessary to touch up areas that need to be included as black in the Thresholded image by using the black Pencil tool. I would also fill in all of the white areas below the horizon so that we might ensure that our Gradient won’t affect them.
Select the White via "Select>Color Range>Highlights".
Create a new layer. Click the "Add A Mask" button on the Layers window. Move the new layer above your photo layer.
Change the new layer to Multiply mode. Add a sky color gradient to the layer.
You might want to apply a Gausian Blur ("Blur>Gaussian Blur") to the Sky mask.