San Francisco used to have more summer fog and extremely steady winds. in recent decades
Winds tend to be more violent and fog is less prevalent.
The same pattern is becoming increasingly common in Southern California and the Columbia River Gorge. The change was reported in many places.
Below are examples from popular articles and scientific journals.
If you want to delve deeper into the science behind it, go to the bottom of the article above, click on the references, and see how science and politics blogs differ.
Even superficial changes require a series of blogs. Therefore, in this blog I will only cover the increasing wind turbulence and decreasing fog at Crissy Field near the Golden Gate Bridge.
recent causal relationship
At a local level, the cause of this crisis field turbulence is the interaction of cold stable northwest westerly winds at the surface with warm unstable northwest westerly winds that hit the Coast Mountains and ripple overhead.
The first graph shows this interaction. Chris is particularly vulnerable to this turbulence because the terrain in upper Marin County is unusually high, complex, and hot.
Winds in San Francisco on June 16, 2024 have red arrows and ovals, representing this strong NNW wind, ≈ 1000 feet (975 MB) aloft.
Notice how this airflow makes the surface winds stronger at some locations while disrupting the winds at other locations, such as Pt. Isabel and Berkeley.
ultimate causation
So why are these NNW winds nearly 1000 feet above sea level (975 MB) or higher becoming more common?
The huge summer North Pacific High is located between Hawaii and the U.S. West Coast. This high pressure helps produce the northwesterly winds common on the West Coast from March to September. It is common for NPH to extend high-pressure lobes, called ridges
Moving toward the Pacific Northwest. This is one of the reasons why the Oregon coast is dominated by northerly winds in the summer, rather than the northwest winds in California.
In recent decades, however, North Pacific high pressure has increasingly pushed the ridge further into the Pacific Northwest and even northern California. Therefore, while there are usually NW surface winds near San Francisco, this ridge produces NNW winds aloft.
Animation shows NPH's isobars roughly parallel to the coast of San Francisco. This pressure gradient creates surface northwesterly winds.
Note how surface isobars kink into ridges as far away as Northern California and the Pacific Northwest.
Also note how this causes NNW winds to develop aloft over the ocean and the San Francisco Bay Area. Since the northwest wind is blowing over the cold ocean, the air is cool and dense (stable air in meteorological terms), so it tends to stick to the sea surface, creating a steady wind.
But when winds aloft blow against the sun-baked Coast Mountains, they heat up and become unstable and turbulent. Sometimes this turbulence produces wind changes, gusts, and lulls when it reaches the ground.
Of course, the real question is…what is the ultimate cause of the increased frequency of NPH ridges. This has to do with Arctic warming.
