Here’s a quick way to do a sky replacement, or at least make the sky transparent in a photograph. Instead of making selections, we can take advantage of blue skies and blue channels. (Remember, photos are generally represented by three channels: RGB—red, green, and blue). Click the lock icon next to the Background layer in the Layers panel to make it a floating layer and not a locked background. Click on the ƒx icon at the bottom of the Layers panel, and select Blending Options. When the dialog pops up, click on the Blend If drop-down menu, and choose the Blue channel. Slide the top-right slider to the left, and as you do, the sky will miraculously turn transparent. Hold Option (PC: Alt) and click on the triangle slider to split it to create a smoother transition. 

This tip originally appeared in Photoshop Tips column by Colin Smith, in the March, 2018 issue of Photoshop User magazine.