In the first half of this year, China saw an average of 4.1 high temperature days where the maximum daily temperature exceeded 35 degrees Celsius (95 degrees Fahrenheit), the highest since 1961, the National Climate Center said in a statement Sunday.

The national average was calculated from the number of high temperature days recorded by weather stations across the country.

China has already experienced four regional heat waves so far this summer, which arrived earlier and have been more widespread and extreme than in previous years, according to the center.

Northern China, a heavily populated region with hundreds of millions of residents, has been particularly hard hit, with more heat waves expected in coming weeks.

So far this year, Shijiazhuang, the capital of Hebei province, has seen 17 high temperature days, followed by Beijing’s 14 days.

Across the country, 110 weather stations have seen record temperatures, the center said.

China’s first heat wave this year arrived on May 28, more than two weeks earlier than usual and affecting 15 provinces.