Attached is a very quick version. I'll note below where to improve it.
Here are the steps:

1. Open both the sky and the foreground image. Only the latter will be
   masked.
2. On the foreground image do Hue/Saturation/Lightness. Press Reset
   and set Saturation to 50.
3. On the foreground image do Image > Split Channel > Split to RGB.
   Close all but the blue channel image.
4. On the foreground image do Layers > New Mask Layer > From Image >
   Source Luminance > Invert Mask Data. This is just a placeholder
   mask. We will replace its contents in a moment.
5. With the mask still active do Select All to be ready for the
   replacement of the mask content.
6. On the blue channel image do Negative Image. This will turn the
   sky dark (which is good because we want it to become transparent
   when we use this image to make a mask). The foreground becomes
   light, which means it will be opaque in a mask. (Note that it
   is unnecessary - contrary to what I implied before - to create
   a mask in this image.)
7. Adjust the current blue channel image with Histogram Adjustment.
   Set the Low Clip Limit to 95 and the High Clip Limit to 129. If
   you boost the height of the histogram using the button with the
   up-pointing triangle you will see that these values fall on the
   left edge of the larger broad peak in the histogram. In this case
   each peak corresponds to some specific content in the image. The
   sky is the spike at the far left. (Because it is a narrow spike
   this is why it is so easy to mask out the sky.) The light part of
   the hill is the small broad peak on the left to the right of the
   spike. The dark foreground is the larger peak on the right.
8. On the blue channel image do Edit > Copy. Make the foreground
   image active (in which you have an active selection on the mask
   layer). Do Edit > Paste > Paste Into Selection. The mask contents
   are replaced with the modified blue channel. You will see good
   separation between sky (transparent) and foreground (opaque).
9. In the Layer Palette for the foreground image make sure the layer
   group (not the mask or the raster layer) is active. Do Edit > Copy.
   Make the sky image active and do Edit > Paste > Paste As New Layer.
   This will paste the layer group from the foreground image over the
   sky image. If you do this you will still be able to adjust your
   mask layer and your foreground layer.

If you want more control over the final appearance try undoing
step 2 after the channel split and try omitting step 7 entirely. Then
after you have pasted the layer group in step 9 you can make the
raster layer active and tweak saturation in a way that you feel best
matches the sky layer. You can also make the mask layer active and
use Histogram Adjustment to give the blend of foreground image and
sky image that you prefer. Instead of following this more intelligent
and flexible procedure, the steps I gave rely on making the changes
ahead of time. I chose to do things this way so it would be obvious
to you what was happening as you went along instead of getting a
useful result only after some mysterious fiddling following the very
last step. Note that you are not restricted to using Clip Limits to
adjust the mask as in step 7. You can alter the balance between how
much light and dark there is in the mask with the Gamma setting. You
can use the Midtone setting to adjust the contrast of the mask, i.e.
whether the transition from light to dark is abrupt (midtones
expanded) or gentle (midtones compressed).

In addition to this bear in mind that you have extra control. If
the exposure for sky or foreground was not perfect you can make
additional tweaks using Gamma Correction, Levels or Histogram
Adjustment. You can do Clarify on the individual layers. (You'll find
this is quite effective in bringing out the detail in the foreground
of your image and there are no halos in the sky because the sky is
masked to be invisible.) You can also do a Copy Merged and paste as
a new layer and Clarify that. You can sharpen either the individual
layers or the merged layer. And so on. It's all remarkably flexible.

Back to the Tutorial

Eleanor's Travels