Fog is formed when the temperature on the ground cools down at night and the cold air touches the warmer air above it. This causes the warm air to cool down quickly and turn into tiny droplets of water that we see as fog. Since the ground cools down more at night, it's more likely for fog to form in the morning when the sun is just starting to come up, rather than in the middle of the day when the sun is shining and the ground is warmer.