China's commercial capital of Shanghai was among dozens of cities baking in scorching temperatures as unusually hot weather buckled roads, popped roof tiles, and drove people to seek the cool in raid shelters underground.

By 3pm today, 86 cities including Shanghai had issued red alerts, the highest in a three-tier warning system.

That level signifies forecast temperatures of more than 40C in the next 24 hours. Construction and other outdoor work are to be halted.

Shanghai, which is still fighting sporadic outbreaks of Covid-19, warned its population of 25 million to prepare for hot weather this week. Since record-keeping began in 1873, it only had 15 days of temperatures exceeding 40C.

In a photograph shared on social media, a Covid health worker in a full-body hazmat suit hugged a one-metre tall block of ice by a road.

Staff at a sprawling 152ha Shanghai wildlife park use up eight tonnes of ice each day just to keep lions, pandas, and other animals cool.

"This year, the heat has arrived a little earlier than before," said Shanghai resident Zhu Daren, as her five-year-old son played at a water fountain.

"Although it is just July, I feel the warm weather has already reached the high point. Basically, you need to turn on the air-conditioning when you get home and put on some sunscreen when you go out."

Three furnaces

China's summer of contrasts this year has brought havoc from heat waves and heavy rainfall. Authorities citing climate change have warned against disasters from mid-July, usually the hottest and wettest time of year.

In a town in the southern province of Jiangxi, a section of a road arched up at least 15cm because of the heat, state television showed.

Nanjing, one of China's three "furnaces" notorious for their searing summers, opened its underground air-raid shelters to residents since Sunday (July 10), with its war-time bunkers equipped with WiFi, books, water dispensers, and even microwave ovens.

The city issued a red alert today.

In Chongqing, the second "furnace", the roof of one museum literally melted, with the tiles of a traditional Chinese roof popping as the heat dissolved the underlying tar. The city called a red alert yesterday.

Chongqing has also deployed sanitation water-spraying trucks to keep its roads cool.

This week, high temperatures, humidity, and ultraviolet radiation are also forecast to envelop the central city of Wuhan, the third furnace, as it is called.

- Reuters