Is there a pre-set or filter in Studio to create fog?
You could probably use the color overlay (or more likely several) with a mask to achieve a sort of foggy effect. Most likely a combination of a gradient mask and some low opacity brush mask.
If you look in the "Texture Effects" category , in the left column, there are a few in the "Ethereal" section, that could be of use.
In texture effects there is a Weathered category. Not the greatest fog look but maybe worth experimenting with?
I would check online to see if you can find a better free texture or create a fog brush or get one free online if you work outside of Studio.
Make sure you don't click on the shutter stock images at the top of the page... they are not free.
One of the easiest ways to do it is with a Color Overlay and a Gradient Filter, then use the opacity to adjust the fog/mist and adjust the brightness in areas of the original to make it look realistic ... here is a quick example, original on the left and mist on the right:
You could do it directly in Topaz Lens Effects - hopefully those tools will come to studio soon...
Thank you ydobemos, John, Don, Ken for all the great suggestions.