The smoke blocked out so much sunlight that a forecast for temperatures to climb to 90 degrees (32 Celsius) instead dipped to the 60s (15.5 Celsius) as though "we're in perpetual morning without the sun to help us warm up," said Drew Peterson, a meteorologist with the National Weather Service's Bay Area office.

Some of the smoke settled in San Francisco Bay and formed a 15-mile (24-kilometer) trail of ash along the tidal line, the U.S. Coast Guard said.

Borrmann said the conditions were expected to linger until Friday. By then, the district expects to issue its 25th consecutive Spare The Air alert requiring residents to cut pollution -- the longest stretch since the program began in 1991. The alerts began Aug. 18 after thousands of lightning strikes ignited three massive wildfires to the north, south and east of San Francisco.

The previous streak was 14 days of alerts in 2018 when the Bay Area was choked by smoke from a fire that devastated the town of Paradise and killed 85 people.

This time, strong winds from the north and northeast pushed smoke from devastating wildfires in the Sierra Nevada foothills and in Oregon and Washington state into lower elevations. A fire that exploded in size Tuesday and prompted evacuation orders for some 20,000 people in Oroville sent the air quality index in Sacramento to a hazardous reading at one point.