California's Central Valley is a 400-mile long expanse, penned in by the Sierra Nevadas to the East and the Coast Ranges to the west. The flatland is composed of remarkably fertile soil, as both mountain ranges erode into and drain out through the valley via the San Francisco Bay. Before the bay was formed roughly 600,000 years ago, geological evidence suggests there was a Lake Michigan-sized inland body of water (referred to as Lake Corcoran in the literature) spanning most of the valley. While the region's paleolacustrine situation is not directly relevant to the present-day weather situation, both are products of Northern California's remarkable topography. Also, the present-day fog is essentially an aerosolized lake, Corcoran reborn in the sky. NASA published satellite images showing the persistence of the phenomenon. This is the tule fog, and it has trapped the Central Valley in a moist, misty prison for nearly a month.
