So let’s talk a little about layer masks. Why didn’t you just make your selection, click on the clouds layer, and hit Delete to remove the part you didn’t want?
Well for a start, who says you won’t want that back later? Masks are a way of non-destructively hiding parts of a photo without permanently deleting them. They work using greyscale alpha transparency values, which probably won’t mean much to you, but simply put that means is that anything pure black is hidden (0% opaque) and anything pure white on the mask will show through the mask (100% opaque). Grey will make things semi-transparent depending on how dark the grey is, where the darker the grey, the less information is shown through.
Now what’s left to do is to balance the image by editing the two layers so that they match a little better. Because these two elements – the sky and the building – are on separate layers I can edit them independently and quite easily.
My first thought is that the building needs a contrast boost, because the sky has some very dark areas as well as some white areas, and the building only goes from dark grey to light grey. To match I’ll boost up the blacks and lower the white point a bit using layers.
So when I talked about masks I also talked about non-destructive editing. We can do this again now while editing the files using adjustment layers, which will give us the ability to come back later and change the values and tweak the image without permanently changing the original photos.
A new layer will always be added just above the one you have selected, and in this case I want to effect only the building layer so I will click on that in the layers panel, and then click on the little black and white circle icon next to the layer mask one that looks a little like an over-simplified ying-yang. This will effect only the building because this layer is above the building, but below the sky layer. For this change you can use a Levels adjustment, a Curves adjustment, or to keep things really simple a Brightness/Contrast adjustment. They all allow different amounts of control, and my preference is Levels.
Here is a before and after of my Levels adjustment, showing both the Layers and Adjustment panels and the image itself: