The smoky cloud cover that has enveloped the Bay Area — and the entire state of California — since Labor Day sent the air quality indexes soaring into the purple and curtailed visibility Friday morning as it hovered like a thick fog.

“It’s exceptionally dense,” National Weather Service meteorologist Drew Peterson said. “It’s blocking out the sun and allowing an even stronger marine layer and fog to move into the area. Unfortunately, that mariner layer is filled with the smoke, and the smoke is mixing with the fog.”

The net result was official and unofficial air readings throughout the region that were shaded purple, or very unhealthy for everyone. A Spare the Air alert issued by the Bay Area Air Quality Management District remained in effect for the 25th consecutive day, a streak that has shattered the old record of 14, set while the Camp Fire was burning in 2018.

By 6 a.m., Vallejo’s official reading of fine particulate matter by the air district was 253, which was actually 15 points below its 2 a.m. reading. Any mark over 200 is considered very unhealthy air for anyone to breathe.

East Oakland (241), San Rafael (236), Laney College in Oakland (228), West Oakland (222), Berkeley (213),  San Francisco (211) and San Pablo (207) all fell into the “very unhealthy” category, as well.

Around the rest of the region, the official air readings showed levels between 151-200, considered unhealthy for all to breathe. Concord’s air measured 186 at 6 a.m., while Redwood City was at 179 and San Jose was at 171.

Readings by Purple Air, a company that measures fine particulate matter through sensors placed throughout the region, showed readings in the area above 300.

The air quality has gotten worse as the winds associated with the marine layer that arrived Monday have gradually weakened. That has caused fine air pollutants that were above the layer to become mixed with the marine layer in an event that Peterson called “turbulent mixing.”

“As that happens, gravity has taken over,” he said. “The particulates have been able to descend to the surface.”

The mixing of the smoke and the marine layer caused the overcast blanketing the region to become so thick Friday morning that the visibility was only one mile at San Francisco Airport, and less than a mile at airports in Concord, Hayward, Petaluma, Santa Rosa, Novato and and Half Moon Bay.

Peterson also said the weather service received reports from observers at the Farallon Islands off the coast of San Francisco that visibility was down to 1/16th of a mile, Peterson said.

“Now that the smoke has filtered into the marine layer, it’s further deteriorating the air quality and visibility,” he said.

Neither figures to improve until Sunday, according to Peterson. Forecasters are tracking what Peterson called a “vigorous” low-pressure system that may act as a cleanser.

“What we’re expecting is an entirely new fresh air mass coming into the region, replacing the one we have,” he said, adding that as the system arrives, “the smoke will not be as concentrated. It’s still going to be everywhere, but it won’t be as concentrated. It’ll be spread out over a larger area, and that should help.”

Check back for updates.