Foggy Inland, Clear at the Coast

Known for it's microclimates, the San Francisco Bay Area weather is a year-round marvel. In the summertime, with little moisture in the Central Valley, the sun often beats down mercilessly on the earth, that in turn heats the air near the surface, the air expands and rises, resulting in an area of thermal low pressure. The summertime pressure gradients between cool, moist coastal areas and the hot, dry Central Valley can be dramatic: It's not uncommon to see a barometric pressure difference of 0.1" Hg or greater between Oakland and Byron, a mere 29 miles away. With the Santa Cruz and Diablo Mountain ranges acting as low altitude barriers, the strong on-shore pressure gradient draws the famous San Francisco fog Eastward, through the Golden Gate and into the bay and other low lying areas.

Winter in The Valley

In the winter months, the effect can be just the opposite. Surface moisture from heavy rainfall combines with low surface temperatures and light winds to form Tule fog in the Central Valley (this is really just a regional term for radiation fog). Add a high-pressure system like the one currently ensconced over North Central California, and you get a temperature inversion that makes the Tule fog persist for days, even weeks. Winter months can often provide an off-shore surface pressure gradient that is just the opposite of the summer pattern. The result is the Tule fog in the Central Valley can be drawn Westward into the Livermore Valley and even into San Francisco Bay.

One benefit of this winter fog pattern is that instrument pilots can depart under visual flight rules (VFR) from Bay Area airports, fly just a few miles to one of several Central Valley airports, and experience instrument approaches to minima - often less than 1/2 mile visibility with indefinite ceilings of 200 feet or less with calm winds. With widespread low IFR, wise pilots and instructors carefully weighs the risks associated with practicing instrument approaches under these conditions.

Seeing an ocean of Tule fog sitting in the Central Valley, but with warm temperatures and clear blue skies aloft is a great way to teach pilots and non-pilots that this type of fog does not burn off. Tule fog will mix into the upper atmosphere and dissipate once the surface winds increase. I still cringe when I hear pilots and weather forecasters (folks who should know better) talk about radiation fog burning off. D'oh!

Here are some recent photographs of Tule fog from above. Having grown up in the Midwest, I'll take Tule fog over sub-zero temperatures and snow shovels, thank you very much.