Lights Dim and Worries Mount as a Heat Wave Roasts California

“The things that affect a fire are wind, relative humidity and heat,” Mr. Foxworthy said. “Heat is the smallest player when affecting fire behavior, but I would say heat is the biggest factor affecting the performance of firefighters.”

He said firefighters in California trained for abnormally hot days and were staying hydrated.

The California Independent System Operator, which manages much of the state’s power grid, ordered rotating power cutoffs for a little over two hours on Friday night to reduce overall demand by about 1,000 megawatts. Bloomberg reported that as many as two million people might have been without power at onetime or another.

Anne Gonzales, a spokeswoman for the power grid operator, said the emergency outage was the result of heat as well as having two power plants out of service. She said that the agency did not expect any shut-offs on Saturday, but that she could not rule out future outages as temperatures remain high.

“We were hoping for a little relief on the demand over the weekend, but these temperatures are holding steady, and understandably, people want relief,” Ms. Gonzales said.

Margaret Barreca was staying cool inside her parents’ home in Sebastopol, Calif., in Sonoma County, when the house suddenly went pitch black. When she looked outside, she saw that her neighbors’ homes had, too.

Ms. Barreca had not been warned of the blackout, but she soon learned that the outage was part of the rotating shut-offs. She spent much of the blackout in her car, charging her phone, until cellphone service went out as well. Many of her neighbors took walks along darkened streets.

“It’s really annoying that the power can just go out, but it’s not just about the power,” said Ms. Barreca, 29, adding that she was frustrated by what she viewed as a lack of action by politicians to slow climate change.