Bay Area weather: Here’s a look at ‘radiation fog’ and how it’s contributing to cold December temperatures

SAN FRANCISCO (KGO) — We have been under a foggy and chilly pattern for nearly three weeks across the Bay Area.

High pressure centered off the California coast has created a resilient pattern of calm winds across the state.

Combined with chilly winter air at night, it is the perfect recipe for what is known as Radiation Fog (the name comes from fog forming under radiational cooling). This is a very common weather phenomenon in the winter and it is also known as “tule fog” in California.

When our winter air cools overnight to saturation, fog forms. The Central Valley acts like a bowl, trapping the fog while it looks for an exit. That exit is the Carquinez Strait, allowing the fog to stream into the Bay Area at an almost unlimited pace.

The constant source of fog is having a chilling effect on our temperatures.

For the first 10 days of December, average temperatures were more than three degrees below average in some cities.