In his latest photography tutorial, noted French photographer (and prolific video tutorial maker) Serge Ramelli demonstrates how to quickly and effectively replace a sky in Photoshop.
As a landscape and cityscape photographer, I'm at the mercy of mother nature. My portfolio is heavily skewed towards dramatic skies — the more fiery clouds, the better. However, as with most things in life, often times things don't go your way. There's nothing worse than planning out a trip to an exotic location only to endure a wall of gray when you arrive. Sometimes, in order to save a shot you need to engage in some post-production wizardry (some would refer to this as "cheating"; I prefer "wizardry").
Compositing a sky is very common. The difficult part is doing it well. As I outlined in a previous article, my method of choice is using luminosity masks. Ramelli uses a combination of the Multiply blend mode, Gaussian blur tool, and simple masks with pretty decent results. Though the final blends he shows in the tutorial lack some realism and have problems with halos and bleed over, with a little more time I have no doubt that these issues can be resolved. Ramelli achieves a good starting point for his blend in a matter of minutes and it's a technique I'll certainly be trying out for my next composite.
If you're just starting out, it's important to build up a good sky library. Keep your camera with you at all times and start shooting!