Fog can occur anytime of the year, depending on the type of fog. In southern Wisconsin, fog is most frequent in the morning, between about 6 and 8 a.m.
Fog is a cloud in contact with the ground, and it forms as the dew point approaches the temperature of the air, increasing relative humidity. The air in contact with the ground can become very humid if it cools or when water from the surface evaporates into it. Either of these processes increases the relative humidity of the air. As the temperature of the air near the ground approaches the dew point, water vapor condenses on tiny particles suspended in the air to form a suspension of small water drops.
Fogs are named for the ways in which they form. Common types of fog in the Midwest include radiation fog, advection fog and evaporation fog.
Radiation fogs form on clear, calm, long nights when the ground and the air in contact with it cool by radiation. As the temperature of the air drops, the relative humidity increases, and fog can form. Early-morning fogs are often radiation-type fogs.
When warm air is advected (blown horizontally) over a cold surface, the air near the ground cools because of energy exchanges with the surface. The relative humidity of the air increases with cooling and an advection fog may form.
Large unfrozen lakes are often shrouded in a fog in the fall and early winter. The lake water evaporates into the air above the lake surface. The air is cooled and moistened, causing the dew point to increase, forming an evaporation fog.
Each year about 700 fatalities occur in the United States because of traffic accidents during fog. A combination of high speed and low visibility is often to blame.