Why is the summer solstice not the hottest day of the year?

Although the summer solstice has the longest day and the highest sun angle, it is not the hottest time of the year. Because the heat is close to the surface, it is still saving at this time and has not reached the maximum. As the saying goes, "the heat is at three volts." The really hot summer weather is calculated according to the summer solstice and beginning of autumn. From mid-July to mid-August, the temperature is the highest in all parts of China.