A thick bank of fog blankets California’s Central Valley. The fog is bracketed by the Cascades to the North, the Coastal Range to the West and the Sierra Nevada Mountains to the East. These high elevation areas are a vibrant green in this image, as they are home to the largest tree species on the planet. Coastal redwoods (Sequoia semperviren) are the world’s tallest trees, reaching over 112 meters (367 feet). They are mostly found in valley bottoms, where fog in the summer occurs on a regular basis and contributes to soil moisture. This particular type of winter fog, or Tule fog, occurs at night when the surface cools quickly; it happens during the rainy season and can persist for weeks. Essentially, all types of fog are clouds that are in contact with ground and can reduce visibility to as little as 3 meters (10 feet) or even to zero in extreme cases. Therefore it is not surprising that Tule fog is a major hazard to navigation and is the leading cause of weather-related accidents in California.
From Modis, which has bigger hi-res versions of that picture.