English: Winter moisture and cool weather created a dense fog over California’s Central Valley on November 13, 2021. The Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Terra satellite acquired a true-color image of the morning fog on that same day.

This fog, known as tule (too-lee), is a frequent feature of winter in the Central Valley. Although gorgeous to view from space, the thick low cloud (fog) can be hazardous for people on the ground. According to local media, the National Weather Service issued a special “very high transportation risk” alert on the morning of November 13. The alert warned drivers to be prepared for traffic delays, reduce speed, use low beams, and allow extra space between vehicles.

The formation of tule fog requires three ingredients: wet ground, still air, and cold temperatures. Conditions are ripe 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. Tule fog season typically begins in December and ends in February.