The weather in Cold Dews
Cold Dew wind is one of the meteorological disasters that befall rice in its late growing period. The Cold Dew is the key time for the heading stage of the southeast late rice. The low temperatures at this time will cause empty, shriveled grain, resulting in reduced production. The damage is called Cold Dew wind. Since the liberation of China, the double cropping rice has gradually expanded northward to the Yangtze River Delta. In these areas, the second rice enters the heading and flowering stage on the second and third ten-days in September, when it is also vulnerable to the cold damage. The name "Cold Dew Wind" has been followed in these areas. In the areas around the middle of the Yangtze River, it is referred to as “She wind” or the Autumn Equinox wind; in the lower reaches of the Yangtze River, it is addressed as “Qiaosui”, meaning “raising ear” or “raising head”. In the Yangtze River valley, there is a proverb that goes, “If the ear doesn’t come out in the Autumn Equinox, the rice has to be cut to feed the cow”. Though the time and address of the damage is different, the basic idea is referring to the damage made by the low temperatures on the late rice.
1. Damages of the Cold Dew wind
In the growth period of the second rice, there are three stages when it is sensitive to low temperatures: first, the initiation stage of the rice head (25-30 days before heading); second, the pollen mother cell meiosis stage (10-15 days before heading); and third, the flowering period. Among them, the rice suffers the damage of the Cold Dew wind in its flowering period with more chance, and seldom in the meiosis stage, but once it comes across the Cold Dew wind in the second stage, the damage is serious. Meanwhile, the initiation stage is essentially free from low temperature damage. In the Meiosis stage, the rice is very sensitive to low temperature. The mainly victims is stamens, leading to the pollen not mature or less. The empty grain or ear deformities, abnormal are following, and the production cuts. Generally, in the year when the damage of Cold Dew wind is severe, rice production is significantly reduced. There are many factors for the Cold Dew, but low temperature is the main one.
In the stage of heading and flowering, if the low temperatures occur earlier, the lower and longer, the damage will be more serious. If it is accompanied with high winds, rain, or excessively dry weather, then the harm will be aggravated. For example, in the coastal areas of southern China, when the cold air southward encounters a typhoon, there will be strong wind, heavy rain, storms or cloudy weather for several days running. It will influence the heading and flowering of the late rice, and cause mechanical damages with more severe harm.
2. Types of the Cold Dew wind according to the damage caused by the Cold Dew wind, the weather can be divided into two types. 1) The wet cold type: the meeting of cold air blowing southward with retreating warm-wet air leads to low temperatures and rainy weather. The results are low temperatures, rain and less sunshine. 2) Dry and cold type: the strong cold air southward brings northerly winds. The wind force is about 3 to 5. The air is dry, sun is bright and the temperature drops significantly. The characteristics are low temperature, dry air, strong wind and a great difference in temperature between day and night.
3. Classification of the Cold Dew Wind The classification of Cold Dew wind bases on the daily average temperature, daily minimum temperature, and rainy days. The classifications are dry-cold type and wet-cold type, and mild type and serious type for each.
4. Defense against Cold Dew wind Cold Dew wind is harmful to late double cropping rice, so active defense measures must be taken: 1) According to the Cold Dew wind law and the safe full heading period of double cropping rice (it is 80% that the double cropping rice would not be influenced by the wind), choose the right seeds. 2) Arrange the production process according to the forecast of the Cold Dew wind. 3) Select the low temperature resistance and high-yielding seeds. 4) Strengthen field management, fertilize intelligently, and irrigate scientifically. Enhance root activity and leaf assimilation capacity to make sure the plants grow well and improve their resistance to the cold. 5) Take the appropriate measures in agriculture to improve the field microclimate.