Most film camera photographers use; UV, or yellow, orange, red, deep red color filters on their cameras with black and white film to increase contrast and cut out haze, you must do the opposite to get; fog, mist, haze, or clouds. You must reduce contrast.
Allot of people try to photograph a low hanging fog, or a mist in the woods, a choreographed scene with smoke generators, etc. but many fail to capture what they originally saw. The problem is that most use by default a SKY-1B filter (strong UV cut), this eliminates the scattering of UV that causes HAZE in the first place. Stronger effects are achieved (in increasing order) when photographers use yellow,orange,red,dark red filters. The human eye, and photographic film both have a hard time focusing on blue light or UV. It actually defocuses the image, that why police, and hunters use yellow or orange sunglasses to eliminate blue light and UV so they get a sharper image down range at their target.
Tiffen and other filter manufacturers make many other filters, for other effects; Smoke, haze, color gradients, etc. These add in effects where none exist, I prefer not to use them myself I dislike the effects. I prefer to use black or white or red wedding veil material to soften an image, with either increase (red) or a decrease(white) the contrast or neutral softening (black).
Here are some manufacturers of filters;
B&W Filters http://www.schneideroptics.com/ecommerce/CatalogSubCategoryDisplay.aspx?CID=57
HOYA Optics http://www.hoyafilter.com/
Please take a look at my other KNOLS;