Fog covered the Upper Mississippi River Valley in late November, 2012. The low clouds caused a significant challenge for travelers, especially those flying out of St. Louis, Missouri and for drivers making their morning commutes. Situated at the confluence of the Missouri and Mississippi Rivers, St. Louis, Missouri was engulfed by low clouds. According to news reports, thick fog descended on the city overnight on November 20-21 and by the morning of the 21st, visibility was reduced to one-sixteenth of a mile (0.1 kilometers). Fog lingered over the region when the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite captured this natural-color image at 10:45 a.m. local time (16:45 UTC) on November 21. Fog filled the Mississippi River Valley as well as the valleys of the Missouri River and many of its tributaries. Nearly all of western Illinois was covered by low clouds, as well as parts of southern Wisconsin, eastern Iowa and eastern Missouri. A bank of low clouds also lingered over Lake Michigan, but skies over Chicago, seen as a gray smudge on the southwest section of Lake Michigan, remained clear. Fog is simply clouds that have formed at ground level. Like all clouds, fog forms when the local air reaches its dew point—the temperature at which water vapor condenses into tiny droplets. In the autumn, the moisture content of the atmosphere remains relatively high, but nights are cool. Overnight, the ground tends to cool faster than the atmosphere, and air in contact with a cooling, sloped surface will slide down and settle in the valley floor. Because autumn brings longer nights, fog has more time to form and is a fairly common occurrence.