October 8 is the first day of Cold Dew, one of the 24 Chinese solar terms, when the sun reaches the longitude 195°. The ancient book, Collected Explanations of the 72 Hou (five days), says that “in lunar September, the cold dew coagulates”. At this time, temperatures are much lower than in White Dew, and the dew is greater in quantity and colder in temperature. The temperature will be low in most areas of China, and there will be less rain. Autumn crops will be ripe.

The ancients treated the dew as a sign that the weather was turning cold. In the White Dew in mid-autumn, the dew condenses and turns white. In Cold Dew, “the dew becomes cold and coagulates”.

At this time, temperatures throughout most parts of southern China continue to decline. The average temperature of Southern China is lower than 20℃. Even in the Yangtze River region, the temperature is lower than 30℃, and the minimum temperature can decline to below 10℃. In the plateau area in the Northwest of China, apart from a few valleys and lowlands, the average temperature of Hou (every five days) is below 10℃. According to the climate science standards of dividing the seasons, it is already winter. In the north, different from the southern area, frost spreads for hundreds miles and snow falls over thousands of miles at this time.

In the period of Cold Dew, rainfall becomes less plentiful in the southern part of China. In the southwest, the rainfall is around 20 mm, and in the southeast, it is 30 to 40 mm. The rain is frequent, day and night in the latter area. It has become a disastrous problem in most areas of South China, since the production of late autumn is very much influenced by this weather. Apart from the rain, the climate is characterized by humidity, more cloud cover, and less sunshine. Also, the number of foggy days increases significantly. The rainfall of autumn influences the speed and quality of farm production. Thus, on the one hand, the farmers should harvest and sow in the sunny days, taking advantage of the weather forecast. On the other hand, taking efficient farming measures, such as digging deep ditches, can reduce the damage of the rain, and improve the efficiency of sowing. In the plateaus, snow in the White Dew period causes the biggest damage. It blocks traffic and endangers livestock production, and should be taken very seriously.

Formula: [Y * D + C]-L

Note: Y refers to the last two numbers of a year, such as 10 in 2010, 08 in 2008, D=0.2422, L=the number of leap years, 21st century C=8.318, 20th century C= 9.098.

Example: The date of Cold Dew in 2088= [88 × 0.2422 +8.318] - [88 / 4] = 29-22 = 7, so Cold Dew falls on Oct. 7.

Exceptions: None.