Hours after sunrise on Wednesday, residents of the San Francisco Bay Area waited for daylight. Instead they got only the faintest suggestion that somewhere above the smoky skies, the sun had indeed risen.
Some called it a nuclear winter. Cars kept their headlights on. Offices towers in San Francisco, where the smoke is mixing with fog, were illuminated as if in the middle of the night.
Across Northern California, giant plumes of smoke from a fire that blasted through the foothills of the Sierra Nevada billowed and spread high in the atmosphere, blotting out the sun.
The Bear Fire, as it is known, added to the smoke already pumped into the atmosphere by the more than 20 large fires burning across California.
Craig Shoemaker, a meteorologist with the National Weather Service in Sacramento, said the immense volume of smoke from the Bear Fire rose overnight to 40,000 feet, an altitude where the air is frigid.
“We have a huge cloud of ash and ice,” he said, adding that the plume resembled thunderclouds.
Fires are essentially creating their own weather, Mr. Shoemaker said. “Without the smoke, it would be a clear day,” he said. “This is all generated from the fires.”
Changing wind patterns will start to push the smoke eastward, potentially clearing the air near the coast, Mr. Shoemaker said, but he added a caveat.
Early in the week, the winds blew wildfire smoke hundreds of miles out over the Pacific Ocean. As they reverse, that old smoke will probably be pushed back ashore.