An ambush is a "three volts". There is a folk proverb, "On the third day of the summer solstice, people's heads fall to the ground", which is the basis for establishing the first autumn. The almanac stipulates: "On the third day of the summer solstice, you will count several volts", which means counting down from the summer solstice and crouching on the third day of the summer solstice. The "Geng Day" here refers to the day when the prefix "Geng" was used in the ancient "Ganzhi Jiri Law".

The so-called "heat is in three volts" can be divided into initial, middle and final three volts, and its date is determined by the date of the solar terms. The climate in dog days is characterized by "high temperature and high humidity", because the southeast wind blows in dog days, and the air is humid. The humidity of the wind causes great humidity in dog days.

The date of "Geng Day" is changeable and not fixed in the Gregorian calendar, so the accommodation date is different every year, but the accommodation date is always between July 1 1 and July 20 in the Gregorian calendar. The fourth Geng day to the fifth Geng day is medium; The first Geng Day to the second Geng Day after beginning of autumn is the last time. Every Geng day is 65,438+00 days apart, and the number of mid-festival days is not fixed. There are four Geng days from summer solstice to autumn, with 65,438+00 days in the middle, five Geng days and 20 days in the middle.

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After lodging, the surface humidity increases, and the heat absorbed every day is more, but the heat emitted is less, and the heat on the surface is accumulated. So it gets 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 greater than that of dry air, which is also an important reason for the sultry weather.

"crouching" means being forced to hide underground, which is the hottest season in a year. Starting from the summer, according to the arrangement of dry and branch days, the third Geng Day is the first month. The fourth Geng Day was in the middle, and the first Geng Day after beginning of autumn was the last. The word "Geng" on Geng Day is the seventh word in ten days of "A, B, C, D, E, Ji, Geng, Xin, Ren and Gui", and Geng Day is repeated every ten days. The start and end time of the dog days is different every year, roughly from mid-July to mid-August.