Original image with no retouching done.

Curves adjustment layer added.

The effect of two cloud layers merged together. You can see that the layers push downward in the center of the image.

The dialog box above shows the layer that I dragged in from the file above. It was a vertical, so it covered only half of this horizontal image. I duplicated that layer and blended the two layers into one.

For the final image I used a motion blur to push the clouds through the canyon. I masked out areas where that technique was not appropriate.

Mist layer is the product of two merged layers. A new mask has changed the entire effect. Again, the colors are from a jpg file and not a png file.

The example below is done with a very simple technique. A new layer is filled with a color and a layer mask that removes unwanted colors from the color layer. Texture can be added to that layer if more detail is required.

First I add a Hue / Sat adjustment layer to add some punch to the original image. Then I made a new layer and filled that layer with a color that I found in the image. I duplicated that color layer and set the opacity to 58% to achieve a 158% fog effect from both of these layers together. The masks on these layers hold back fog from specific areas in the image.