In the winter time we have the cold working for us and against us when it comes to fog. In most situations the cold air is much drier and will offset any moisture in the air. The best chance of seeing fog is when we get a warm air mass in place over an existing snow pack. This is also known as advection fog, as the warm air is advecting (moving), in. The melting of the snow will add moisture to the air and the evaporation process itself, along with the cold snow, will lower temperatures near the surface and create fog.
Radiation fog also forms on a clear cool night. This fog appears mainly on a clear night with calm winds. The earth will lose its heat to space and cool quickly under these conditions. If there is a deep enough layer of moisture at the surface, the cooler conditions near the ground will help form fog.
Pay attention to the temperature and the dew point. The dew point is the temperature at which we need to fall to for the air to become 100% saturated, forming fog. The lower the dew point the drier the air and vice versa. The closer the temperature and dew point, the better chance of seeing that fog appear.
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