As the summer sun scorches California's giant Central Valley the air heats up and rises, creating a lower pressure area under the column of rising air. Altamont Pass, at 1,009 feet of elevation, is a low point in the hills and mountains ringing the central valley. In comparison the Sierras to the East tower up to 10,000 plus feet, the Tehachapis down South and coastal ranges to the west reach 3,000 plus feet.
As the heat builds to over 100 degrees during the day the valley sucks air over Altamont Pass to fill the void left by the rising heated air.
If the valley causes wind, how does that cause San Francisco Fog? As air is drawn over the pass from the San Francisco bay area this in turn pulls in moist air from the Pacific ocean. When this moist air moves over the particularly cold waters of the California current it quickly cools. Our California current is particularly cold because it comes down from up north off Canada and Alaska. Thanks to the Coriolis effect northern hemispheric ocean currents rotate clockwise, which is why the East coast has warm waters, sea cows and hurricanes and we have cold waters, sea otters and fog.
So what happens now that our cold current has chilled the air heading in to San Francisco Bay through the Golden Gate? We have all seen cold water dripping down the outside of a cold bottle pulled from the fridge on a hot day, thanks to condensation. As the air hits the cold bottle it's temperature falls and lower temperature air can't hold as much water, so mist forms on the cold surface. Similarly, when the California current drops the temperature of the moving air water condenses in the air and mist starts to form on the surface of the water. For a while condensation and evaporation may duke it out, but on foggy days condensation wins.
I think we see the same mist effect on the surface of a lake in the morning, here's San Pablo Reservoir on my morning commute.
To reach the density of fog the water vapor needs a catalyst on which to coalesce or condense, something like dust or pollution or, somewhat surprisingly, salt crystals. Let's call these necessary fog catalysts hygroscopic condensation nuclei for short. Waves breaking along the coastline throw salt crystals into the air, and this high concentration of salt crystals provides the condensation nuclei the freed up water molecules need to coalesce and form a ground-level cloud, or fog. So on summer days from a high vantage point you can often see a hug fog bank offshore stretching to the horizon, and as the day progresses a finger of fog will creep through the Golden Gate and stream toward Berkeley.
Geography comes into play again. Our iconic Golden Gate is a 1 1/2 mile wide sea level gap in all the hills that run up and down our coast, so it is the easiest path for the fog to move from the ocean towards the valley.
Like Sisyphus the fog flows toward the valley but never reaches it, day after day. Once you get past the first hills it's too dry to sustain the fog. With the hot headwinds I too sometimes feel like Sisyphus going nowhere on a trainer stuck in the barrel of an industrial hairdryer. But, unlike our Greek antihero, I will eventually get over the hill to my house on the bay shore which is always cooler than San Ramon. That's nice when temperatures in the valley get over 100 degrees.
That regular headwind also created conditions for a forest of wind turbines (aka windmills) to grow on Altamont Pass. You see turbines all over the hills going from Livermore to Tracy on 580 or Patterson Pass Road. The fact that this wind increases as the day heats up in the central valley is great for the economics of wind energy because it correlates wind generation with air conditioning, and A/C drives summer peak load. Peak power is the most expensive electricity to generate. All the cheap generation sources are already going flat out, now you have to turn on those plants you keep on cold standby to run just a few hours a year. Peaker plants are often basically jet engines mounted on a concrete pad, with the hot exhaust creating steam to make electricity. Cheap to build but expensive to run.
Wind-powered electricity economics normally suffer from intermittency - you can't turn then wind on when you need it, you just take it when it comes. If you can get wind-power at the time of peak that's a big bonus. If your wind blows at night when your nuclear and coal plants are throttled back and potentially even dumping energy to avoid a 1 or 2 day shutdown due to minimum downtime, the payback for the wind power is not good.
Siting electricity generation sources close to consumption centers minimizes transmission losses. Combine that with highly-correlated electrical loads and generous federal and state incentives and Altamont Pass wind farm became the world center for wind power development toward the end of last century. US Windpower/Kenetech was one of the largest manufacturers, making turbines right here in Livermore.
But after a decade or more of relatively low cost fossil-fueled electricity and California's expensive lessons in corporate exercise of market power* in a deregulated market for electricity, leading wind energy manufacturers are now in countries like Denmark and China where the government supports the industry.
You could theoretically power the whole USA with wind turbines in just a few windy states like Wyoming and Montana, using pumped storage (the electrical Sisyphus) and other technologies to buffer wind's intermittent output. However those generation sources are too far from consumption centers, moving the energy to consumers would be prohibitive with transmission losses, wheeling constraints, and other issues. You need government policy, something like a Tennessee Valley Authority, so we could be investing in the USA and become less reliant on tinpot dictators in -Stan countries or anachronistic theocratic monarchies awash in oil and gas.
If you ride San Ramon or Livermore valleys you know the wind is always there, we just need to put it to use.
* possible future post, but politics is a slippery slope and maybe that downhill is a bit too sketchy on a bike blog
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