The direct cause of heat waves is the heat dome phenomenon.

The heat dome phenomenon is a phenomenon in which high pressure developed in the upper-middle and upper atmosphere at a height of 5 to 7 km above the ground forms a hemispherical roof, trapping hot air and causing a heatwave. That is, the high pressure developed in the sky acts as a lid and the hot air stays in a certain area one after another to prevent the hot air from being discharged.