Bay Area gets a double dose of fog: marine layer vs. tule fog

By CBS Bay Area

It was a foggy start to Sunday across the Bay Area — the foggiest part of California on this Nov. 23.

But isn’t summer normally our foggy season? Why are we seeing so much fog this morning?

Well, there are two different types of fog that impact the region throughout the year! They are known as radiation fog, aka tule fog, and advection fog, aka marine fog, and they were both present in the Bay Area on Sunday.

Radiation fog

Tule fog, which occurs in the central valley of California on calm, clear nights after rain, is a type of radiation fog. Radiation fog forms overnight when the ground cools by radiating heat into the sky, and this cooling then cools the air just above it to the dew point, causing moisture to condense into fog. Tule, radiation, fog is more likely in the winter, when the state sees the most rain. Because of the storms that dropped a lot of rain last week, there’s still a lot of moisture on the ground to create the thick tule fog.

Advection fog

The marine layer, sitting off the West Coast, is a type of advection fog. Advection fog forms when warm, moist air moves over a cold surface, causing the air to cool and condense into fog. This more often occurs during the summer, when upwelling in the Pacific brings cold water to the surface, creating the marine layer.

Since we are not in summer or winter, we are getting a mix of the summer-winter fog. Now, next time there is fog outside, you’ll be able to say exactly what kind it is and what’s causing it.