Normally we need damp ground, clear skies at night, and light winds for fog to form. During the cold season, melting snow is also a key ingredient.
Water from the melting snow seeps into the ground and then evaporates. This process puts a lot of water vapor into the air just above the ground. At night, as temperatures cool this causes the water vapor in the air to condense into water droplets. Light winds allow this process to continue, eventually leading to fog forming.
If you encounter fog during the cold season it is not just reduced visibility that you will need to keep in mind. Overnight and early morning temperatures are usually below freezing. If fog develops, then freezing fog will be a concern. The water droplets in the air can condense back onto surfaces, including the ground, and freeze to create potential slick spots.
-Meteorologist Ronelle Williams