Fog/Mist effect

Open the image you'd like to add a fog/mist to, and create a New Layer . I will be naming mine fog base.

Make sure your foreground and background colors are black and white. You can hit D to reset them to this.

On layer fog base, you'll want to create some clouds by going to Filter >> Render >> Clouds. You want a pattern that doesn't have too much white or black but a nice balance of both. You can keep rendering clouds until you're satisfied with the pattern.

Set the Blending Option of layer fog base to Screen (in the Layers window or Layer >> Layer Style >> Blending Option).

It should now look like this.

We want to spread out the fog/mist a bit. Let's start by giving it a bit of texture. Go to Filter >> Artistic >> Poster Edges. The default options are fine (Thickness at 2, Intensity at 1, Posterization at 2).

Select the Smudge tool . (If you only see the Blur or Sharpen tool, hold that down for more options.) Select a fairly large brush and set the pressure to 50%.

Now have fun with the fog and smudge it around! Simply hold down on an area and then move your mouse to the side, smudging the fog. Do this to several areas in horizontal motions until it's smoothed out.

Select the Eraser tool and choose a brush with faded edges (like an airbrush). Erase off any areas you don't want covered by the fog.

Make another New Layer . I will name it fog overlay.

Render clouds on fog overlay by doing Filter >> Render >> Clouds again.

This new cloud layer will help break up the fog/mist more. Set the Blending Option of layer fog overlay to Overlay or Soft Light.

And that's it! You can smudge it around more or erase some areas until it's to your liking.