It has been hotter in California in the past few days than it has been in living memory.

In Fairfield, a small town about 50 miles northeast of San Francisco, the thermometers at the Travis Air Base weather station reported a temperature of 47 degrees.

The value broke a 1913 heat record for Northern California, which is normally cooled by a fresh sea breeze.

Because of the record temperature, it was almost as hot in the small town on Monday as it was in Death Valley, where 53 degrees were measured.

The cause of the heat wave in California is an extensive high pressure area over Nevada and Utah that has been stable for weeks.

It causes air from great heights to sink toward the ground, becoming increasingly warmer as it does so.

The high air pressure at high altitude puts a lid on the hot air near the ground that has spread over increasing areas of the American West over time.

Almost everyone in California and Nevada is groaning from the heat wave, as well as people in large parts of the states of Utah, Oregon and Arizona.

While summer temperatures of up to 40 degrees are common in the Nevada deserts and California's Central Valley, the current heatwave is extremely rare for the populated Pacific coastal regions around San Francisco and Los Angeles.

The network agency expects for the coming afternoons that Californians will consume more than 51 gigawatts of electricity - that is more than all power plants, solar and wind systems can provide.

The operator of the power grid has therefore called for voluntary self-restraint.

If the voluntary power saving does not succeed, the network agency has announced that entire districts will be shut down for several hours.

The last time Northern California was plagued by such extreme heat was in 2017. But then, the high temperatures only lasted for three days.

However, the current heat wave has lasted for more than a week.

Some relief isn't expected until next weekend as Hurricane Kay, currently raging over the Pacific Ocean off the coast of Mexico's Baja California Peninsula, slowly moves north.

Even if it loses some strength over cooler water, its high-altitude winds are still strong enough to displace the sustained high over California.