We took a day trip to Mt. St. Helens in February, while it was actively venting. It was an overcast day and the fog from the volcano merged into the clouds of the overcast. While watching the mountain, it didn't seem like it ever changed, but I took a video with my digital still camera and it quite clearly ends differently than it began.
updated: I dug up this photo because it echos the weather we've been having recently: rather cold. In the early mornings and evenings, we have localized fog. That damp, chilly, clinging fog that is featured on the moors of Sherlock Holmes.