Folks are often perplexed by our local ocean fog. Even though it is tempting to conceive that it has actual evil intent (we just saw it stay offshore until just before 4th of July fireworks were scheduled to go off, where it rushed onshore!), it happens for reasons that are pretty easy to understand.
The Pacific Ocean is a relatively cool body of water, which moderates our coastal environment, keeping it from becoming too hot in summer as well as too cold in winter. Our interior valleys, far from this influence, do experience much warmer summer temps (as well as colder winter lows). As this warm interior air rises (orange arrows in the image at left), it creates a vacuum that pulls in air from surrounding regions. Because of the convenient gap in our coastal mountain ranges at the entrance of San Francisco Bay, this replacement air comes through the ‘Golden Gate’, pulling the ocean fog along with it (blue arrows).
The ocean fog is there because of the difference between the cooler Pacific ocean (due to upwelling cold water from ocean depths) and the relatively warmer air temperatures. This is the same reason that the Atlantic coast of Portugal, and the Pacific coast of central Chile, both have a similar fog effect, and why you seldom see fog on the coasts of the Mediterranean Sea, which is a warmer body or water.
Sights of fog spilling over our coastal hills are very familiar to long time residents. Sometimes when a warm inversion layer holds the fog down, this cool air can only come in through the lowest points. The main ‘low point’ along this part of California coast is the Golden Gate, which is why it is so picturesquely half-shrouded in fog. Where does the fog go as it spills through this opening? First stop is Berkeley, where we live! I work in Oakland, which often warms up during the day in July, but back home in Berkeley, the fog can sit all day. My tomatoes are not particularly happy right now, and the rose is developing mildew, and many other warm-loving plants are in a sort of suspended animation, waiting for the occasional warmer day. But those that DON’T like the heat are enjoying a longer season that otherwise!
Predicting when the fog will come has often been elusive, especially to the layman. Either you get caught unprepared like the many San Francisco tourist who we see shivering in their shorts and tropical shirts brought especially for ‘sunny California’, or we take no chances and drag around various layers of clothing that can warm us if the fog suddenly engulfs our current location. Weather reports are seldom specific enough to be of much use, but recently a friend directed me to this SFGate.com page which shows the immediate fog forecast via an animated .gif (shown at right), complete with date and hour. I’ve been referencing it lately and find that it has been very accurate – a real use for me as I travel between Oakland and Berkeley each workday – two regions with distinctly different fog patterns!