05 Add the gradient to the mask
What I’m trying to do is make the mist appear stronger in the distance, so I drag upwards from a point near the bottom of the picture to another point about half way up – I’ve circled the start and end points for the gradient in this screenshot.
You can see the gradient on the thumbnail image for the layer mask in the Layers palette. The black part of the gradient (at the bottom) is masking the solid white layer, the white part (at the top) is showing it fully, and the gradient in between is blending in the white layer progressively.
This simple trick instantly makes the mist effect look realistic.
06 Darkening the sky
This next part isn’t really necessary, but it can give the picture a suitably heavy, overcast effect and it’s a chance to show the ‘multiply’ blend mode in action.
For this I’m going to create a new solid color adjustment layer, but this time I’m setting the colour to black.
07 Set the blend mode to multiply
Blend modes change the way layers interact with those below, and they can be accessed using the drop-down menu at the top of the Layers palette. When it’s set to multiply mode, the new layer will darken the image below rather than simply covering it up.