Imagine the Golden Gate taking one full breath each year. Due to differences in air pressure between the California coast (high pressure) and the Central Valley (low pressure), air flows eastward in summer and westward in winter through the San Francisco Bay.
The inhalation causes the summer fog for which the Bay Area is famous. The gradual exhalation begins as seasons change. In winter, a high-pressure system called the Pacific High moves away from the coast. The difference in temperature between the Bay Area and the Central Valley becomes small during the day and much greater at night. Inland air cools as it comes into contact with cold ground, resulting in an atmospheric inversion: two distinct layers of air, with warm on top of cold. When the inversion is accentuated, thick tule fog forms and flows toward the coast through San Francisco Bay. By March, tule fog is rare in the Bay Area.
The term tule is a Native American word for bulrush, a kind of plant that grows in freshwater marshes. Wetlands once decorated the Sacramento and San Joaquin River floodplains in such abundance that fog blanketed the valley floor during the rainy season. Today the low, dense fog veils the fields and freeways of the Central Valley, causing more automobile accidents in the state than any other weather phenomenon.