Cold Dew (Hanlu) is the seventeenth of the twenty-four solar terms and the fifth solar term of autumn. It occurs when the Big Dipper points to the celestial stem xin and the sun reaches the ecliptic longitude of 195°, typically falling on October 7–9 in the Gregorian calendar. Cold Dew is a deep autumn seasonal marker and the beginning of the xu month in the sexagenary stem-branch calendar. Similar to White Dew, Cold Dew is also a solar term that reflects the characteristics of climatic change; during the Cold Dew season, the dew is thicker and the cold air is more intense.

The “Twenty-Four Solar Terms,” which originated in the Yellow River basin, are a product of ancient agricultural civilization. They represent a knowledge system formed by ancient forebears who, attuned to the farming seasons and through observing the movements of celestial bodies, recognized the changing patterns of season, climate, and phenology over the course of a year. Cold Dew is the first among the twenty-four solar terms in which the character “寒” (cold) appears. The Yueling Qishier Hou Jijie (Collected Explanations of the Seventy-Two Pentads of the Monthly Ordinances) records: “Cold Dew is the solar term of the ninth month. The dew qi is cold and about to condense.” As the dew droplets grow frigid and verge on freezing, they emit a faint, cold shimmer — hence the name “Cold Dew.” Cold Dew has three pentads: in the first pentad, wild geese arrive as guests; in the second pentad, sparrows enter the great water and become clams; in the third pentad, chrysanthemums bloom with yellow splendor.

Common weather phenomena during Cold Dew include fog and haze, Cold Dew wind, and the autumn rains of West China. After Cold Dew, the days gradually shorten while the nights lengthen, sunshine decreases, cold air periodically sweeps southward, the diurnal temperature range widens, and autumn dryness becomes pronounced. Most of China comes under the control of a cold high-pressure system, bringing the rainy season to an end. Under this cold high pressure, the days are warm and the nights cool, with the daytime often presenting crisp, clear autumn skies. Climatically, the south enjoys refreshing breezes and cool, dry weather with little rain, while the northeast and northwest regions of the north already exhibit winter scenes. Traditional customs during Cold Dew mainly include admiring red maple leaves, eating sesame seeds, eating crab, and drinking autumn tea. During the Cold Dew period, winter wheat sowing in the north should be completed without further delay to avoid yield loss. In the south, rapeseed and broad beans should be planted at the appropriate time. In South China, a type of hazardous weather may occur that directly affects the progress and quality of the “Three Autumns” (autumn harvest, autumn plowing, and autumn sowing). Therefore, weather forecasts should be utilized to seize clear days for harvesting and planting. After Cold Dew, health preservation emphasizes drinking plenty of water and keeping warm.