When wildfires are pumping out smoke, the slightest shift in the wind can push sooty air from one place to another.
What happened in San Francisco and along the Peninsula Tuesday morning is a perfect example of this.
The prevailing winds shifted from a northwesterly direction to more southerly around 5 a.m., carrying smoke from the CZU August Lightning Complex in the Santa Cruz Mountain toward the city, according to the National Weather Service. In prior days, the CZU Complex smoke moved toward Monterey County.
The smell of smoke was strong along the Peninsula and the city this morning and air quality quickly reached "unhealthy" levels.
When air quality reaches "unhealthy," the air districts advises against exercising outside and says you should remain indoors entirely if you have respiratory disease such as asthma.
San Francisco recorded an AQI of 159 at 11 a.m. and Redwood City 156, according to the Bay Area Air Quality Management District.
The Air Quality Index operates on a scale from 0 to 500. The higher the AQI value, the greater the level of air pollution and the greater the health concern. An AQI value of 50 or below represents good air quality, while an AQI value over 300 signals hazardous conditions.
The Air District pulls its AQI data from sensors approved by the state, while the increasingly popular Purple Air app uses data from sensors people install outside their homes.
The smoke along the coast and bay front and in San Francisco tends to be heaviest in the morning, with conditions improving in the late morning to afternoon when the ocean breeze picks up. The winds push the smoke east, and this is when the East Bay becomes socked in with sooty, choking air. This scenario is expected to occur again today, with the coast likely getting some relief this afternoon. BAAQMD meteorologist Duc Nguyen said the ocean breeze often kicks up around 2 p.m.
The prevailing winds are expected to switch back to a more northwesterly direction on Thursday and this could help San Francisco see better overall air-quality throughout the entire day.
But there's one caveat, said Nguyen. "The Woodward Fire in Point Reyes National Seashore has been intermittently emitting smoke, especially in the afternoon, when it’s hotter. With a northwesterly regime, you may get some impact in the later afternoon. If the smoke from that fire continues to be more active, you get the plume coming down."
Will conditions improve any time soon?
Widespread smoke is expected through the week as three major groups of fires —CZU Complex in Santa Cruz Mountains, LNU Lightning Complex in North Bay and SCU Complex in East Bay and Central Valley — continue to burn. Containment is increasing, but Nguyen said overall containment stands at about 30% and air quality won't improve until fires are more than 60% contained.
The Air Quality District has a Spare the Air alert in effect through at least Friday, warning of the high amount of particulate matter pollution in the air.
"It is illegal for Bay Area residents and businesses to burn wood or manufactured fire logs in fireplaces, wood stoves and inserts, pellet stoves, outdoor fire pits, or any other wood burning devices," the district said. "In addition to not burning wood, you can help improve air quality by working remotely and driving less every day."
The district advises residents to go inside if they smell smoke outdoors. And keep windows and doors closed.
