Realistic Fog

Step 1

Open any picture on which you want to add a fog.

Step 2

Create a new layer and name it fog.

Step 3

Take the Brush Tool (B), set its size to 50px, under Mode choose Dissolve and set Opacity to 50%. See the picture below.

Under Foreground color choose some grey tone, try to get fog color :),  and start paint the part of picture which you want to be fog.

Step 4

Select the fog layer and click on Filter > Blur > Gaussian Blur. Then in Gaussian Blur panel under Radius set 20px. See the picture below.

Your picture now looks like this:

Step 5

Select the fog layer again, and click on "Add layer mask" icon in layers palette. Look at the picture below!

Step 6

Take the Gradient Tool (G), and in Gradient Editor set Black and White color.

Step 7

Drag the Gradient Tool from top - to the picture, to create a gradient to the sky and remove the fog on the top.

Finally, our work looks like this:

We're done!

Have a nice day!