Radiation fog is contributing to unusually cold temperatures across the San Francisco Bay Area this December [1].
These weather patterns are significant because they create localized cold pockets that differ from the region's typical coastal moderation. The phenomenon affects daily commutes and energy demands for residents facing a sharper drop in temperature than usual.
According to meteorologists, the current chill is the result of specific atmospheric conditions. Clear skies and calm winds allow heat to escape rapidly from the ground, which leaves a layer of cold, dense air trapped near the surface [1]. This process creates the radiation fog currently seen across the region.
Temperatures in parts of the Bay Area have dropped into the mid-40s °F, which is approximately seven °C [1]. This cooling effect is most pronounced during the nighttime and early morning hours when radiational cooling is at its peak.
John Smith, a meteorologist, said that radiation fog forms when clear skies and calm winds allow heat to escape rapidly from the ground, leaving a layer of cold, dense air near the surface [1]. The lack of cloud cover acts as a missing blanket, allowing warmth to radiate into space rather than staying trapped near the earth.
Residents have reported a noticeable shift in the early December air. The combination of humidity from the fog and the low temperatures creates a damp cold that can feel more piercing than dry cold air.
While the Bay Area is known for its fog, radiation fog differs from the more common advection fog that rolls in from the Pacific Ocean. Advection fog occurs when warm, moist air moves over a colder surface, whereas radiation fog is a localized event driven by the cooling of the land itself [1].
“Radiation fog is contributing to unusually cold temperatures across the San Francisco Bay Area this December.”
The occurrence of radiation fog highlights the impact of localized terrestrial cooling on urban microclimates. Unlike the broader marine layer that typically regulates San Francisco's temperature, this phenomenon demonstrates how specific wind and cloud conditions can override the ocean's stabilizing influence, leading to sudden, sharp temperature drops in inland and valley areas.
