On November 15, 2021, the sky over San Francisco was covered with thick clouds and fog.

There were no major wildfires in California, but the air quality index reading for the San Francisco Bay Area on Monday morning was higher than 100 on Purple Air, a popular website that provides real-time air quality readings. 

On Monday morning, the area was a sea of ​​orange dots. It's the same on weekends.

The readings of the Bay Area Air Quality Management Zone show a better situation. The air quality in the Bay Area is mostly below 100, which is in the yellow "medium" range-but this may still make residents wonder why the air quality in winter is not good, especially It was raining for a while.

Aaron Richardson, spokesperson for the management district, said that it is not uncommon for air quality to deteriorate in the winter from November to February due to smoke from the fireplace. Regional data shows that there are approximately 1.4 million fireplaces in the area.

Richardson said: "We have an inversion layer in the area, hot air covers the cold air, which forms a cover that can capture the smoke from the fireplace," and pointed out that the inversion layer is a typical winter weather pattern in the Bay Area.

Another factor that affects the situation on weekends and Mondays is offshore wind blowing particulate matter from the Central Valley to the coast, causing deterioration of air quality in the Bay Area.

Richardson said: "This is the season when we start to see the accumulation of fine particulate pollution. Due to the offshore airflow from the Central Valley, our pollution is a little bit more than usual."

Krstine Roselius, another spokesperson for the management district, said that wind is also pushing humidity-these conditions will push up the readings of what Roselius calls the humidity-sensitive Purple Air sensor. 

"We have fog from the Central Valley, and humidity affects the purple air sensor," Roselius said Monday morning. "Now, I'm looking at Purple Air, San Francisco and Marin have numbers over 100. On our website, San Francisco is 85. Purple Air uses different technologies. They run rates [about 30% higher than the sensors used Area]."

The Air Quality Index (AQI) ranges from 0 to 500. The higher the AQI value, the higher the degree of air pollution and the greater the health problems. An AQI value of 50 or below indicates good air quality, while an AQI value of more than 300 indicates a dangerous situation. Purple Air and air quality districts use this scale.

The air quality area extracts its AQI readings from high-precision sensors monitored by the Environmental Protection Agency. Roselius said the Purple Air sensor is installed by ordinary people and connected to the company's application.

"These people bought these and put them in their yard," Rosellius said. "If you put it in the dryer vent in your backyard, the humidity will throw off the sensor. If you grill in your backyard one day, it will throw it off."

Regardless of whether you look at the purple air or the air zone, the air quality in the Bay Area is worrying. As far as management is concerned, Chasen said it is not serious enough to require air quality warnings for health hazards.

The aviation zone forecaster said that the weather is expected to be mild in the coming days.

Amy Graff is the news editor of SFGATE. She is a native of the Bay Area and started her career in journalism at the Daily Californian newspaper of the University of California, Berkeley, majoring in English literature. She has been working at SFGATE for 12 years. You can email her at agraff@sfgate.com.