I like working in Photoshop (and want to keep expanding my skill set), so I found this tutorial on creating a fiery sky with a comet coming in over the city. Here's the URL: http://www.photoshoplady.com/photoshop-tutorial/turning-a-systemic-city-into-fire-scenery/
I'm not completely happy with how it turned out. The overall effect is pretty good, if you don't look too closely. This tutorial was a bit vague on the explanations, just presenting the steps and settings for the various adjustment layers, and not always specifying which layer they were working with. I would have liked it to explain more WHY; that would be helpful when using different images, as I was. I especially had issues when it came to adding the fiery clouds. There are supposed to be exploding particles around some of the buildings, but even though I downloaded the brush set they were using, I couldn't seem to get the same effect. So I did learn some effects, but I feel like I didn't get the whole picture. Maybe I bit off more than I could chew, with this one!