"Fu" is a folk saying, which means that Yin qi is forced to hide underground by Yang qi. In ancient China, the "trunk-branch calendar method" was popular, and 60 groups of different names matched by 10 heavenly stems and 12 earthly branches were used to record dates and recycle them. "Summer solstice, three Geng's head falls", which is the basis for establishing the first fall.
The date of Geng Day is changeable and not fixed, so the date of lodging is different every year, but the date of lodging is always between July 1 1 day and July 20.
Hottest cause
After lodging, the humidity of the ground surface becomes larger, and the heat absorbed every day is more, but the heat emitted is less, and the heat on the ground surface is accumulated. Therefore, it is getting hotter every day, and when it enters the dog days, the accumulated heat on the ground reaches its peak, and the weather is the hottest.
In addition, there is much rain in summer, the air humidity is high, and the heat capacity of water is much larger than that of dry air, which is also an important reason for the sultry weather. In July and August, the subtropical high intensified. Under the control of the subtropical high, the downward flow inside the high pressure made the weather sunny and partly cloudy, which was conducive to sunlight, and the ground radiation warmed up, making the weather even hotter.