California Central Valley fog, poor Bay Area air quality explained

By CBS Bay Area

It’s been very foggy across Central California this winter – in fact, there were 22 consecutive days of fog in December, making it the longest stretch of non-stop foggy days since the 1970s. But why has it been so foggy? 

Let’s start with the record-breaking rain across central and southern California from September through November last year, which was 300-400% above average. Above-normal rainfall creates the potential for thicker-than-normal fog. After all that rain, a very stable area of high pressure developed over the state, which acted like a lid to trap all that moisture in the Central Valley. 

The valley acts like a bowl, so on cold winter nights when winds are calm and skies are clear, thick valley fog forms. With no storm systems to disperse the fog, it just sat there getting thicker day after day.

So high pressure helped form and trap the fog in the Central Valley, and an inversion layer is keeping it there. Normally, it gets colder when you go up in the atmosphere – it’s usually chillier on the top of the mountain than in the valley below. But an inversion layer occurs when the opposite is true: cold air is trapped at the surface beneath warm air. Since warm air rises and cold air sinks, this puts a “lid” on the atmosphere; the cool air near the surface cannot escape. The air just sits still, stagnating and allowing the fog and air quality to worsen.

So it’s the inversion layer that not only traps fog and bad air quality across the Central Valley, but also traps bad air quality across the Bay Area. The next time you can see that line of haze in the sky, you’ll know there’s an inversion layer set up, and temperatures are actually getting warmer the higher up you go.