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When I came back from a visit to Hadrian's Wall, I was quite disappointed to see the result:

The sky is overexposed, but there's still some colour in the sky, so all is not lost. Open the picture in Photoshop. First, go to image > calculations Select red, normal and new document. (If you try green and blue you'll see that the sky only gets whiter.)

Then go to image > adjust > selective color and make it blue. Select neutrals and adjust the sliders until you have a nice blue sky. Try these settings:

Now drag this picture onto the original picture and select multiply and blend it.

The sky's OK now, but the ground was affected too, so we'll need to fix that.

Select the eraser tool, and set the size to 999. Let the opacity be around 40-50%

Notice that I didn't remove the horizon completely, because if I did there would be a noticeable "halo". Better to let it be a little dark, something which also adds to the horizon's contrast.

Finished!

Be aware that when you multiply a photo onto itself, noise may be enhanced. Depending on the quality of your original photo, you may have to blur the new layer. However, if the image is going to be downsized afterwards, then the noise will go away in the process.