The dog days appear between the light summer heat and the heavy summer heat, and are the days with the highest temperature, humidity and sultry heat in a year. The so-called "dog days" refers to the "dog days" of the lunar calendar, which is the hottest period of the year.

Dog days, one volt is ten days, which is calculated according to the number of days. Therefore, the dog days are divided into three days, that is, the Geng Day on the third day after the summer solstice is the beginning day of the first volt.

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.