You might also be able to do something with geothermal vents. If you have a cold climate with a lot of moisture, and there is a source of fairly continuous heat upwind of your valley, you might be able to sustain cloud cover more consistently.

I don't have time to think about that very deeply, and my understanding of meteorology is fairly weak to start with, but to my uneducated mind, that seems plausible.

And yes, this thread would be more appropriate in the General, or General and Miscellaneous Mapping, or possibly How Do I?? forum. I'm sure a mod will be by in a few hours to relocate it.