why there is fog in winter?

Answer 1

Cold air gets more easily saturated than warm air. When saturated the watermolecules will form drops of water, which is what fog essentially is.

Of course air humidity is the other factor

Answer 2

We get allot of fog around here in spring and fall. With so much snow on the ground, as the weather turns warmer, the air becomes saturated with moisture from the melting snow, as it tries to evaporate into the air. If there is a heavy cloud layer above, that will keep the fog low and heavy, as it becomes fully saturated, until the sun can break through and help to dry up and evaporate the molecules of water in the air.

Hope that made a bit of sense.