Fog is pretty simple in concept. It’s a cloud that’s really close to the ground. Fog forms when the surface temperature and dew point are close together – around 5° F or so.
California has a few different types of fog; there’s Tule fog, endemic to the Central Valley. There’s the coastal fog of San Francisco, which has a name and a Twitter account (@KarlTheFog) – this is also the fog that waters the coastal redwoods. There’s also “June gloom”, a Southern California phenomenon helped along by the Catalina eddy, occurring in the late spring and early summer.
Of these, I think the northern coastal fog is the most famous – “the coldest winter I ever spent was a summer in San Francisco”, which is usually misattributed to Mark Twain. However, the Tule fog is the most notorious – it’s responsible for the occasional hundred-car pileup along I-5, and is California’s leading cause of weather related traffic accidents.
Tule fog happens when the ground cools through a clear sky at night, causing a temperature inversion, which is the state when lower air is cooler than higher air. The fog is kind of self-sustaining as it keeps the sunlight from warming the ground, which cannot warm the air, which doesn’t lower the humidity and dissipate the fog. Further, since the Valley is surrounded by mountains, the fog can be trapped and persist for days. There’s a good explanation in the Los Angeles Times (and explanation of the effects of climate change) here.
My wife and I live near a freeway (as many people in Southern California do) and I think I can hear more freeway noise when it’s foggy. Of course Wikipedia has an explanation for that. It could also be due to the fact that the wind is generally calmer then, too.
Fog is not what people from out of state think about when they think of California but it is one thing that California means to me. It keeps things warmer in the winter and cooler in the summer. It waters the coastal redwoods and the Central Valley crops. Fog is a break from the continual sunshine (and the seemingly more frequent heat) of Southern California. An occasional gray day makes the bright days seem brighter. Even if I hear more traffic noise.