Chicago is anticipating mostly cloudy weather overnight, with patchy fog expected to develop by morning. This pattern, typical for the region during this time of year, results from cooler temperatures and high humidity contributing to fog formation. Residents should anticipate reduced visibility, especially in the early morning hours, as the cloudy conditions persist without significant precipitation. Fog forms when air near the ground cools to its dew point, causing water vapor to condense into tiny droplets. Clear skies, light winds, and high humidity enhance this process, common in Chicago during fall and winter.