How Bay Area fog forms, helping make San Francisco a cold summertime city
We deal with it every year; some know it as the marine layer, others call it Karl the Fog, but either way, it's what helps make San Francisco the coldest major city in the US during the summertime.
But how and why does it form? Let's zoom to the coast.
First, let's take a look at how the fog forms. It happens when the temperature at the surface drops to the dew point. The dew point is basically a measure of how much moisture is in the atmosphere, and it's always lower, or the same, as the temperature.
So, for a very simplified example, say the afternoon high temperature is 63°, and the dew point is 50°. As the sun sets, the dew point increases, and the temperature drops. If the temperature cools to the dew point, fog will begin to form.
So, that's how fog, and clouds in general form. But for California specifically, cold ocean water gets upwelled along the West Coast. Westerly winds over the Pacific push that cold air so it clashes with the warm air over land, rapidly cooling down the temperature to the dew point, creating fog!
The thickest fog happens when there's the biggest difference between the ocean and land temperatures.
And it's not just the upwelling along the West Coast that keeps us especially foggy; there's also what's known as a Pacific Highs that develops every summer and is a huge factor in our weather. High pressure creates sinking air, trapping all the fog that's trying to rise at the surface. The westerly, onshore winds then disperse the fog inland through the valleys. When the fog encounters an obstacle that's taller and bigger than it is, it will pool up next to it until it finds a way around or just keeps getting thicker.
This type of fog can actually happen when air moves over any relatively cold body of water — I've seen it plenty of times over Lake Michigan in Chicago, where I grew up!
That's why the Central Valley's fog can get so thick, the fog is trapped by the surrounding mountains and keeps getting thicker. Same process for the creation of fog, but the reasons for the fog in the valley vs. along the coast are different.