The fog during winter season is more on the day when sky is clear.

Fog consists of visible cloud water droplets or ice crystals suspended in the air at or near the Earth's surface. This is a type of fog that forms when cool air passes over a warm body of water or moist land. In general we see that fog is formed whenever there is a temperature difference between the ground and the air. When the humidity is high enough and there is enough water vapor or moisture fog is sure to form.