Bailu means that when the white dew comes, it means that the sultry heat of summer is over and autumn has truly entered.

Bailu is the 15th solar term in the "Twenty-Four Solar Terms", the third solar term in autumn, the end of the Shen month and the beginning of the Unitary month. Dou refers to Gui; the sun reaches 165 degrees of yellow longitude; the festival is on September 7-9 in the Gregorian calendar. "Bai Lu" is an important solar term that reflects the growth of cold air in nature. As the cold air turns from defensive to offensive, it is still hot during the day when the sun is shining, but the temperature drops quickly after dusk, and the temperature difference between day and night gradually widens.

When it comes to white dew, the summer monsoon is gradually replaced by the winter monsoon, and the cold air The shift from defense to offense, coupled with the southward shift of the direct sun point, shortens the daylight hours in the northern hemisphere, weakens the light intensity, and rapidly dissipates heat from ground radiation, so the temperature decrease gradually accelerates. The white dew has basically ended the sultry heat of summer, the weather is gradually getting cooler, and the cold brings dew condensation.

According to the "climate average temperature" standard proposed by modern scholar Zhang Baokun, the period when the climatic average temperature is 10-22°C is spring or autumn. According to this standard, starting from the Bailu solar term, autumn begins in various places. A folk proverb goes: "The body is bare and the body is bare." ’ It is said that the weather is calm and you can give clothes and ears. "It is intended to remind people that it is still hot during the day, but the weather is already cool in the morning and evening. If you go bare-chested, you will easily catch a cold.

The twenty-four solar terms represent the changes in natural rhythms in the calendar and establish the "December" The specific seasons of "Building". There are four o'clock in the year, three months in spring, summer, autumn and winter, and two solar terms in each month. Each solar term has its own unique meaning.

Weather changes

After entering the Bailu solar term, the summer monsoon is gradually replaced by the winter monsoon, and the cold air moves southward more and more frequently. In addition, the position of the sun directly hitting the ground moves southward. The sunshine time in the northern hemisphere becomes shorter and the intensity of sunlight weakens. It is often clear and cloudy at night, and the ground radiates heat. It is fast, so the temperature drop is gradually accelerating. There is a saying that "the white dew comes on the autumnal equinox night, and the night is cooler."

After the white dew, the temperature difference between day and night gradually widens. Although the temperature is higher at noon during the day, it is colder in the morning and at night. There is a hint of coolness. According to the standard of "climate average temperature" proposed by modern scholar Zhang Baokun to divide the four seasons, the period when the average temperature is 10-22℃ is spring or autumn. According to this standard, starting from the Bailu solar term, various places have begun to enter. Autumn. At this time, the temperature difference between day and night in various parts of China can reach 8℃-16℃, so Bailu is a solar term with a large temperature difference in the year.