Why is SF’s air quality so bad right now?

Update: The Bay Area Air Quality Management District decided Thursday afternoon not to issue a Spare the Air alert for Friday, saying they expect pollution levels to stay in the mid-to-high moderate range.

The hazy skies and poor air quality that San Francisco residents started noticing Wednesday is due to a relatively normal winter weather phenomenon called a “temperature inversion,” not wildfires or an unusual amount of pollution, weather experts said Thursday. 

Aaron Richardson, the Public Information Officer for the Bay Area Air Quality Management District, said he was unsure how long the poor air quality would last and said that the district only issues Spare the Air alerts when the federal standards are exceeded.

“We are in the ‘Moderate’ category now, not ‘Good’ anymore,” he said, calling the current situation “typical” for this time period.

While the poor air quality is affecting the whole Bay Area, San Francisco appears to be hit hardest. On Thursday morning, air quality was reading between 150 and 200 on the PurpleAir website and 100 on AirNow, prompting residents to take to Twitter and contact the National Weather Service.

On their Twitter account, the National Weather Service said the bad air quality was due to a weather phenomenon called an “inversion.”

Normally air temperature decreases with altitude, but in a temperature inversion in the winter, cold temperatures cause the ground and air closer to the ground to lose heat quickly at night. In response, warmer air rises and acts as a lid, trapping the colder air—and pollution—close to the ground. This means the air that we breathe, which is the air layer closest to the ground, gets more polluted because particles from cars or fireplaces can’t dissipate, according to the European Environmental Agency.