San Francisco is famous for its fog. Perhaps no place on Earth (certainly not in the United States) is more closely associated with fog. The Farmer’s Almanac lists San Francisco as the third foggiest spot in the United States behind Point Reyes, Calif. and Cape Disappointment, Wash., and all three of these locations have the same thing to blame for their foggy days: the Pacific Ocean.
The Pacific Ocean is cool — literally. The chilly water temperatures are courtesy of the California current, which moves north to south, originates off the coast of southern British Columbia and flows south along the west coast of the United States, eventually ending off the Baja California Peninsula. When warm, moist air rides over this cold water, the air cools quickly to the dew point, which causes condensation and formation into a shallow cloud layer: fog.
The fog inherent to San Francisco is a specific type of fog, advection fog, meaning it has horizontal movement. Unlike other types of fog, such as valley fog or radiation fog, which tend to stay in place, advection fog moves from one location to another. This is why it can resemble a rippling blanket moving over the Golden Gate Bridge.
While many may find the persistent fog mind-numbing and depressing, it actually has some benefits. Nicknamed “nature’s air conditioner,” it can provide cool relief during warm days for a region where millions do not have air conditioning.
Fun fact: That same cool and stable air that creates the foggy days in San Francisco is the same reason why thunderstorms aren’t all too common in the Bay Area. In fact, according the National Weather Service in San Francisco, the region averages just six days with thunder per year.
Now, San Francisco isn’t the only city to have a name for a recurring weather phenomenon. Travel south, toward Los Angeles, where NBC L.A.’s morning meteorologist told me recently that the marine layer responsible for the “May Gray” and “June Gloom” across Southern California this time of year also has a name — Ela! You can follow her on Twitter, too, via @elamarinelayer. Californians know how to have fun with their weather!
Lastly, a few words from Karl and Ela, your resident tweeting weather phenomena!
Weather is awesome. #cwgpicoftheweek