Linda I used a couple of different fog brushes that I downloaded for free from Brusheezy for PS. Using brushes to create it on a separate layer allows me to control the opacity and color of the fog as I apply it as well as the opacity of the layer. Fog is not always white. I can change the direction or orientation of the brushes in the Brush Preset Picker. I can also erase or remove it from specific locations without effecting my original image.

In this particular image I wanted to try and match the brush to the shoreline but I didn't want fog on all the water in the foreground. In real life the closer you are to fog the lease dense it is so from the viewers point of view there would be less fog at the bottom of the photo. The fog brush I was using applies the fog in a slight gradient. Using it straight out of the box would have made the fog thicker at the bottom of the photo and too thin on the shoreline. I flipped the brush 180 degrees using the Brush Preset Picker and applied my fog. I could then erase the thicker fog from the trees. I have included a screen shot of the fog applied in black to a white layer. I applied it first to the bottom of the page then flipped the brush 180 degrees and applied it to the top of the page. I did it at 100% opacity so you could see how it has a slight gradient at the edge.

I hope someone finds this information helpful.

Keni