Pictures of the red and orange skyline flooded social media as some reported oversleeping because the sky was so dark.
The strange colour is due to light being filtered through smoke from California’s wildfires, said regional air district meteorologist Jarrett Claiborne in a video shared on the Bay Area Air District’s Twitter page.
“It looks like the scene from a movie,” he said as he explained what caused the eerie colours.
“When we have particles in the air they can act as filters and can ‘scatter’ certain colours out.
“With smoke for example, that’s filtering out other colours allowing red, orange and yellow to reach the surface.”
One social media user shared a picture that suggested waking up to the apocalyptic sky was like waking up in hell.
Another shared footage of what it was like driving across the Golden Gate bridge at 9.20am in the morning.
Local police forces advised drivers to keep their headlights on all day and the National Weather Service has said conditions could worsen throughout the day.
The wildfires are California’s worst on record with nearly 2.5 million acres burned already this year.
Historically the worst of the wildfire season does not begin until the autumn.
Since the middle of August, fires in California have killed 11 people, destroyed more than 3,600 structures, burned old growth redwoods, charred shrubland and forced evacuations in communities near the coast, in wine country and along the Sierra Nevada
Daniel Swain, a climate scientist at the University of California, Los Angeles, conservatively estimated the fire had burned about 400 square miles in 24 hours.
“The unbelievable rates of spread now being observed on these fires — it is historically unprecedented,” he tweeted.
The US Forest Service, which had taken the unprecedented measure of closing eight national forests in Southern California earlier in the week, ordered all 18 of its forests in the state closed Wednesday for public safety.
In Paradise, where 85 people lost their lives and nearly 19,000 buildings were destroyed, the sky turned from black to cherry red and ash carried on strong winds rained down in a scene reminiscent of the fateful morning of November 8 2018, former Mayor Steve “Woody” Culleton said.
“It was extremely frightening and ugly,” Mr Culleton said.
“Everybody has PTSD and what not, so it triggered everybody and caused terror and panic.”
People in foothill communities east of Los Angeles were warned to be ready to flee, but the region’s notorious Santa Ana winds were weaker than predicted.
“We’re encouraged that the wind activity appears to be dying down,” Governor Gavin Newsom said. “The rest of the week looks a little more favorable.”