The Tiffen Fog filter mimics the effect of natural fog, creating a soft glow and flare and producing a warm romantic tone. 
 

A natural fog causes lights to glow and flare.Contrast is generally lower, and sharpness may be affected as well. Fog filters mimic this effect of atomized water droplets in the air. The soft glow can be used to make lighting more visible, make it better felt by the viewer.The effect of humidity in, say, a tropical scene can be created or enhanced. In lighter grades, these filters can take the edge off excess contrast and sharpness.Heavier grades can create unnatural effects, as for fantasy sequences. In general, however, the effect of a strong natural fog is not produced accurately by Fog filters in their stronger grades That is because they are too fuzzy, with too much contrast, to faithfully reproduce the effect of a thick natural fog. For that, Double Fog filters are recommended.
 

Double Fog filters have milder flare and softening characteristics than standard Fog filters while exhibiting a much greater effect on contrast, especially in the stronger grades. A very thick natural fog will still allow close-up objects to appear sharp. So will a Double Fog filter.The key to the effect is the much lower contrast combined with a minimal amount of highlight flare.