In St. Louis, Missouri, what type of fog would you expect to find forming during winter?

In St. Louis, Missouri, during winter, valley fog is a common phenomenon because it tends to form in low-lying areas. This type of fog develops when moist air cools and condenses in valleys as temperatures drop overnight, especially during clear nights with calm winds. The cold air settles into the valleys, and if the humidity is high enough, condensation occurs, leading to dense fog.

While radiation fog and steam fog can also occur in certain conditions, they are less likely in this context. Radiation fog typically forms on clear nights after the ground loses heat, cooling the air close to the surface, and is more common in flat, open areas rather than in valleys specifically. Steam fog occurs when cold air moves over warmer water, creating a mist effect; however, winter temperatures in St. Louis generally lead to conditions not conducive to this type of fog unless there are specific water bodies involved. Upslope fog occurs when moist air is forced up a slope, which is not a predominant feature in the relatively flat surroundings of St. Louis in winter.