I usually take pictures of a fog just before sunset, but I know it's not always the best time, because the land is often dry by then. It is much better to get up in the morning. Then in the wetland areas the probability of hitting the fog is almost a hundred percent, and you have a chance to capture the low light (if you're lucky to have both the fog and the sunshine - where I live it's a rather a rare occassion!). Howver it's worht remebering that the source of light must not always be the Sun. Ordinary street lights in favorable circumstances, can give an interesting effect in a photograph.