This visible satellite image, taken by NOAA's GOES orbiter at about 900 AM CDT, shows areas of fog that formed in the overnight and early morning hours across the Midwest and Great Lakes area. This "radiation" fog forms overnight with clear skies and light winds as the earth's heat radiates out to space. Air near the ground cools to saturation causing fog to form. Under these conditions, fog is more likely in low areas such as river valleys, and over open farm fields, where there may be slightly higher moisture content in the air. In this picture there is fog in the Wisconsin and Mississippi River valleys, over southern Lake Michigan, northeast Missouri, and northwest Ohio.