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.
So what causes this spooky advection fog? And why does it often peak in summer?
The Pacific Ocean is cool — literally. The chilly water temperatures are courtesy of the California current, which moves north to south. This current originates off the coast of southern British Columbia and flows along the U.S. West Coast, eventually ending off Mexico’s Baja California Peninsula. When relatively warm, moist air rides over this cold water, the air cools quickly to the dew point, which causes condensation and a shallow cloud layer of stratus.
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. Plus, this fog is key to the success and deliciousness of California wine!
There’s a reason it tends to be prevalent and thick in and around San Francisco during the summer, and the reason is somewhat counterintuitive. We can thank heat. Although high heat is generally associated with high pressure, extreme temperatures will cause air to rise and this can help create a local low pressure zone. In and around San Francisco, this “heat low” helps pull the fog inland on the really hot days. Just don’t tell folks on the beaches near San Francisco that it’s hot!
Here’s another fun fact about San Francisco’s fog. Did you know it has a name? It’s Karl, and boasts a mischievous Twitter account called @KarlTheFog. Let’s see what he’s been up too, shall we?
In addition to its name, and the idea of Fogust, there’s also the “June gloom.” You might get the sense that Californian’s have a special relationship with coastal fog.
Weather is awesome. #cwgpicoftheweek