Fire season has been getting longer in Silicon Valley, and with it are smoke and air quality issues. The Bay Area is full of micro climates, and that translates to areas which are more or less impacted by these problems.
- windier areas, where the breeze blows from the San Francisco Bay or the Monterey Bay to inland areas may have the cleanest air if the fires generating the smoke are far away
- lower elevation areas may have less smoke if the blazes causing the smoke are far (distant fires seem to cause the smoke to be higher up)
Weather patterns, smoke and air quality
Please note, I’m not a meteorologist, but am going to share my observations from living here most of my life (except a few years from 18-23, I’ve been in this area).
Our Silicon Valley weather is dominated by the nearby Pacific Ocean. Most of the time, the coast enjoys foggy mornings and sunnier afternoons, and the inland areas are impacted by that. When the coastal overcast burns off, clouds recede from the Monterey Bay. In the mid to late afternoon, winds reverse, and breezes pick up. This is also true with winds coming off the San Francisco Bay and moving south along the Calaveras Mountains.
Wind patterns are key for the discussion around smoke and air quality.
Most of Silicon Valley is in the Santa Clara Valley. Valleys can trap air in what is called a heat inversion at times (a warm area prevents the air below it from moving around – we see this in winter sometimes). When that happens, we get a “Spare the Air” day and are asked to do what we can to help the air pollution problem. The coastal areas don’t have this issue, so usually don’t have smog. They have plenty of wind from the ocean. If there is a fire elsewhere in California, most of the time it does not impact the coast to the same degree as the inland areas.
As a general rule, when the fog is pushed in from the Pacific, it comes through any low points or passes it can find. The foggier areas to the north of Silicon Valley, such as Daly City and San Francisco, don’t offer much resistance to the push from the Pacific. Nearly always, coastal wind means cleaner air.
The easiest way to spot those low points or mountain passes is to find the roads going over the coastal range on a map: Highway 17 in Los Gatos to Santa Cruz, Highway 9 in Saratoga to Ben Lomond and Felton, Highway 152 in Gilroy (Hecker Pass Highway) for Watsonville, Highway 84 from Woodside toward San Gregario, and Highway 92 to Half Moon Bay.
Where these roads empty out inland, it will be windier than the areas in protected valleys (of which there are many not really too noticeable on a map).
Downtown Los Gatos can get very blustery at times, which east Los Gatos on the far side of Blossom Hill is relatively protected. (Campbell also gets the wind as it follows the Los Gatos Creek, but it seems to dissipate a little as it moves toward San Jose.)
If the wind is clearing the smoke out of downtown Los Gatos, it may be lingering in areas without that coastal blast. A quick check at purpleair.com will tell you if that is the case or not.
As I write this post, there are a number of fires burning in northern California – thankfully not near Santa Clara County this year. The smoke has drifted from the Sierra Nevada Mountains out toward the coast in recent days. Today we have some strong breezes and wind coming in from Santa Cruz, and it’s pushing the smoke further inland. A few days ago, though, we had smoke and air quality that was pretty poor most of the day. It began clearing up when the marine layer moved back toward Santa Cruz and the winds from the Pacific forced their way through the passes. It was clear that those areas catching the coastal winds were getting much better air quality long before the rest of the county saw improvement.
When the fires are far, the smoke is high
Something else to be aware of is that if the forest fires are far away, the smoke will be high up, not close to the ground. In recent weeks I’ve noticed that when the air quality was good on the valley floor, sometimes it was quite bad up in the Los Gatos Mountains or Santa Cruz Mountains.
When the fires are near, all bets are off on smoke and air quality
It was just a year ago that we had infernos raging on both sides of the Santa Clara Valley. There was the CZU Fire Complex in the Santa Cruz Mountains and the SCU Fire in the east foothills. (You can view a fire map from last August on my Live in Los Gatos Blog.) Smoke filled the air both here and along the Monterey Bay and Pacific Ocean and anyone living near the hills and open space was thinking about evacuation plans as much as the smoke. I hope we never have another episode like that again.
The typical wind patterns can change from friend to foe if there’s a blaze nearby.
For the cleanest air, you may want to consider living somewhere windy, such as near the San Francisco Bay (Redwood Shores and Foster City are really right over the bay), the Monterey Bay, or near one of the passes from the coast to the SF Bay.
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