The traditional East Asian calendars divide a year into 24 solar terms. Bailu (pīnyīn) is the 15th solar term. It begins when the Sun reaches the celestial longitude of 165° and ends when it reaches the longitude of 180°. It more often refers in particular to the day when the Sun is exactly at the celestial longitude of 165°. In the Gregorian calendar, it usually begins around September 7 and ends around September 23.
When the White Dew comes, the weather is turning cold. Although sunshine in the day is still hot, after sunset, temperatures decrease rapidly. At night, water vapors in the air turn into small water-drops when it encounters cold temperatures. These white water-drops adhere to flowers, grass and trees. When the morning comes, sunshine makes them look crystal clear, spotless white and adorable. Therefore, people have given them this beautiful name, "White Dew".
There is a saying which goes, "The nights of White Dew and the Autumnal Equinox period are becoming colder and colder." This is because summer monsoons have been replaced by winter monsoons, and north winds are more common. Cold air moves frequently to the south. In addition, the duration of sunshine in the northern hemisphere is shorter than before. There are few clouds in the sky at night, so heat radiation from the ground is very fast. Therefore, temperatures decrease quickly.
