Formation
Tule fog is a radiation fog, which condenses when there is a high relative humidity (typically after a heavy rain), calm winds, and rapid cooling during the night. The nights are longer in the winter months, which allows an extended period of ground cooling, and thereby a pronounced temperature inversion at a low altitude.
In California, tule fog can extend from Bakersfield to Red Bluff, over 650 km (400 miles) distant. Tule fog occasionally drifts as far west as the San Francisco Bay Area, even drifting westward out the Golden Gate, opposite to the usual course of the coastal fog.
Tule fog is characteristically confined mainly to the Great Central Valley due to the mountain ranges surrounding it. Because of the density of the cold air in the winter, winds are not able to dislodge the fog and the high pressure of the warmer air above the mountaintops presses down on the cold air trapped in the valley, resulting in a dense, immobile fog that can last for days or at times for weeks undisturbed. Tule fog often contains light drizzle or freezing drizzle where temperatures are sufficiently cold.
Tule fog is a low cloud, usually below 2,000 feet in altitude and can be seen from above by driving up into the foothills of the Sierra Nevada to the east or the Coast Ranges to the west. Above the cold, foggy layer, the air is typically warm, dry and clear. Once tule fog has formed, turbulent air is necessary to break through the temperature inversion layer. Daytime heating sometimes evaporates the fog in patches, although the air remains chilly and hazy below the inversion and reforms soon after sunset. Tule fog usually remains longer in the southern and eastern parts of the Central Valley, because more winter storms affect the northern Central Valley.
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