Kids have a water fight on Sunday in East China's Jiangsu Province. Photo: VCG

Many Chinese cities entered Sanfu on Sunday, a term from the Chinese ancient Stem-Branch calendar that is used to describe the hottest days of summer.

As of 6 am Sunday, Chinese meteorological authorities issued the yellow alert for heat waves in parts of the country where the highest temperature is hitting over 35 C. China has a four-tier weather alert system, with red representing the most severe, followed by orange, yellow and blue.

When the yellow alert is issued, citizens are advised to reduce outdoor activities. The days are characterized by high temperature and humidity, Wu Rui, chief service officer at the Shanghai Meteorological Bureau, told the Global Times on Sunday. He reminded citizens to "eat more fruit and vegetables, maintain a regular life routine and take precautions when going outdoors."

Around 2 pm on Sunday at the Bund of Shanghai, the Global Times reporter noticed some tourists taking pictures and walking along the Nanjing Road despite the unbearable heat. A vendor on the Bund said that the stand sells around 1,000 bottles of drinks a day during hot days.

Cleaners, policemen and sellers were still working in the heat. This year, sanfu starts from Sunday and will end on August 19, lasting 40 days.