tommykjensen has a topic in Crtique about replacing a sky, here. I have some general tips and comments about the process - but not his image - and I didn't want to hijack his thread so I thought I'd start a new one here in Post Processing.
If you ever think that you might wish to replace a sky, one suggestion that I've got is to build a "Sky Library." When I'm out and I see a beautiful sky but no picture I'll just click off several frames of the sky. Aim the camera level and take frames while turning in a circle to get the sky and clouds with various light angles. Don't aim up, don't aim down too much - take the images as a natural angle. Don't worry if you capture too much land at the bottom - chances are that you'll only use the top half of one of these images anyway. Take the images with your normal landscape lens.
By staying natural and turning in a circle you'll capture natural clouds, and hopefully the light in one of them will match an image taken some other time. If you normally take landscapes at 17mm don't expect clouds at 100mm to look natural. If you normally shoot near dawn then don't expect a noon cloud shot to match the lighting.
One of the most important points of "sky swapping" is matching the light (which also means shadows) on the 2 images.
Here's an old image I took. This is straight from the camera through Capture 1. I adjusted contrast a bit in C1, but not fully. I didn't touch the color, and this has no sharpening.
(This, by the way, is the first "decent" picture that I ever took. It's CRW_0231.CRW - 230 failures and finally I got something decent. ROFL!)
Not so great, huh?
The picture has a boring overcast sky, but I liked the picture and the way the sunrise light up the land. And I happen to like this particular piece of the planet Earth.
So I found another picture with a good sky - one that I took about 30 or 40 minutes after this picture, so I still had nice early-sun lighting on the clouds. It was in RAW so I changed the white balance to match the dawn light better, and developed it. I then erased the original sky fairly simply - Magic Wand, and Delete. This left a lot of edges that I erased with the pencil. Painstaking. There are better ways, but none so exact. I hate being anal about this stuff...
The new sky was too evenly lit, so I added a left-to-right gradient mask to the clouds so that the right side was darker and matched the rightmost house better.
I enhanced some of the windows and trim so that the sun was glinting off them - only a couple, and subtly. I added a couple of seagulls to the sky. There are some in the foreground - though they're not really visible in this small image - so adding more over the sky adds some continuity.
The original ocean was a lousy gray since it was reflecting the original overcast sky. So I selected the ocean using a mix of Color Range and Magc Wand. I then de-selected anything not ocean using Alt-Lasso. (This was done rough, really, as I didn't need precision for this step.)
Then I saved the selection as a channel, giving me a mask of the ocean. I then cut&pasted that channel into a new image, flipped that image (Image... Rotate... Flip Vertically) so that I had a mirror image of the mask. I cut&pasted that back into the original image as a new channel, and moved it so that I could see only clouds through the mask. I then used that channel mask to make a copy of the clouds. Again another round of cut&paste into a new image, flipped it, cut&pasted it back into the original.
I now had a mirror reflection of the clouds in the shape of the ocean. (Phew! I wish I knew an easier way of doing that!) I made that reflection into a new layer, and lined it up roughly with the ocean, and then changed the opacity to 7%. This left the ocean looking like it was reflecting the new sky. It's subtle, because the ocean's so dark, but the ocean just didn't look right when it was a drab solid gray. Now it's lightly reflecting the new sky.
I went through a simply painstaking process on the leafless tree that sticks up - the one about 1/3 from the right. I knew that if this didn't match right it would stick out like a sore thumb, so I spent some time on it. I erased most of the non-branch pixels using the Pencil tool. I knew that the continuity - merging the land into the new sky - would help the final image a bit. And after spending so much time on that tree I simply pruned the one next to it. LOL! That was too much work.
EDIT: After looking at the picture a bit I remembered something that I had done with that tree... I Lasso'd around the tree, Cut it out and then Pasted it to a new layer. This great reduced the number of colors, allowing me to more easily use Select Color Range and the Magic Wand to remove the gray pixels of the original boring sky. When done I changed the contrast dramatically, lightening the gray pixels and darkening the tree. I then reduced the Opacity of this layer until the tree looked right. This made for a much better blend against the new sky.
The final result:
So the tips:
Build a library of good skies.
Match the light. Noon clouds won't match an early morning scene.
Match the Shadows!! Add some if necessary.
Match the colors - a sunset sky won't reflect blue in the ocean!
Add some continuity from land to sky. Those little touches lend believability to the final image.
Other tutorials from the folks here can be found in this topic.
