08 Adjust the opacity
Full black is a little strong for this effect, so I’m going to reduce the black solid color layer’s opacity to 50%.
09 Add another gradient mask
Finally, I’ve added a gradient to this layer’s mask too, but in the opposite direction – again, I’ve circled the start and end points on the screenshot. This blends in the darkening effect right at the top of screen so it looks as if there is heavy cloud overhead.
Incidentally, if you find your gradients have gone in the wrong direction, high ctrl/command-Z to undo the gradient and drag it out again in the opposite direction.
10 The finished picture
I think this has worked pretty well. Normally, if you’ve got some perpendicular object right in the foreground, like these gates, it can work even better if you mask them out on the ‘mist’ layer, so that they don’t get ‘mistier’ higher up. It’s not always easy to get this exactly right, though, and in this instance I tried both approaches and decided this simpler one worked best.