English: Wintertime in California’s Central Valley means snow in the high Sierra Nevada and a blanket of tule fog hanging low over the Valley floor. The Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Terra satellite acquired a true-color image of Central Valley fog on December 20, 2020. This relatively narrow bank stretches about 325 miles (523 km) from north to south. Tule fog is an exceptionally dense fog that forms in the Central Valley each winter after the ground has become damp from a recent rain. During the day, the warmth of the Sun causes the moisture to evaporate into the air. After sunset, air temperature slowly drops. Because cool air can hold less moisture than warm air the many tiny moisture droplets suspended in the air during the day combine into large droplets, creating “low cloud”, or fog. Tule fog tends to form—often rapidly— after sunset, become thickest just before dawn, then slowly thins as the air warms in the morning.