BEIJING -- Beijing, sweltering in extreme heat, is expected on Saturday to top 40 degrees Celsius (104F) for a third day, with sizzling temperatures already baking an area the size of California in northern China.

In Beijing between 1990 and 2020, the average number of days with temperatures of 35 degrees Celsius or more was 10.6, the official Beijing Daily reported, citing official data.

June is not yet over and that number has already been beaten, the newspaper said, after temperatures in Beijing surpassed 35 degrees Celsius for the 11th day this year on Saturday.

Parts of Hebei, Henan, Shandong, Inner Mongolia, Tianjin and Beijing either raised or kept their hot weather alert at "red", the highest in China's four-tier warning system.

In the colour-coded system, red is the most severe, followed by orange, yellow and blue. A red alert signifies the temperature could exceed 40 degrees Celsius within 24 hours.

People eat ice cream amid a red alert for heatwave in Beijing, China June 23, 2023. Photo: Reuters

As of 1:13 pm (0513 GMT), a combined area the size of California - or 450,000 sq km (174,000 sq miles) - had recorded temperatures of over 37 degrees Celsius, according to local media.

"Last year's heatwave gives some sense of the risks to China's food supply and the potential impact on prices," Capital Economics wrote in a note on Friday.

"Another drought would hurt crop yields while livestock are vulnerable to high temperatures. Even if a similar outcome is avoided this year, climate change means such events are likely to happen with increasing frequency in future."

The heatwaves, the second round in about 10 days, were caused by warm air masses associated with high pressure ridges in the atmosphere. The effect was amplified by thin cloud cover and long daylight hours around the summer solstice, according to Chinese meteorologists.

People shield themselves from the sun, as they walk amid a red alert for heatwave in Beijing, China June 23, 2023. Photo: Reuters

On Friday, Beijing baked in temperatures as high as 40.3 degrees Celsius, after sizzling at 41.1 degrees Celsius on Thursday, the second-hottest day recorded by the Chinese capital in modern times.

Until this week, the city of nearly 22 million people had never logged two consecutive days above 40 degrees Celsius since setting up its main observatory in the southern suburbs in 1951.

Beijing's all-time high of 41.9 degrees Celsius recorded on July 24, 1999, remains intact for now.

On Friday, Beijing authorities said schools can reduce or even suspend classes if the weather becomes very hot.

The heat waves in northern China are expected to abate by Monday before regaining strengthen later in the week.