上海地区从9月上旬开始,由于北方冷空气活动加强,温度逐步下降。根据上海近五十年(1925年—1974年)气早的年份,如1932、1957年出现在9月4日;晚的年份,如1945、1948年则推迟到10月上旬;但比较多的年份集中出现在9月16～20日之间,平均十年中有四、五年会遇到低于17℃的低温。过去上海地区讨论后季稻“翘穗头”问题时,仅仅考虑 Shanghai area starts from the beginning of September, the temperature drops gradually because of cold air activity in the north. According to Shanghai nearly 50 years (1925-1974) angry early years, such as 1932, 1957 appeared on September 4; late years, such as 1945, 1948, was postponed to early October; but more Year appears concentrated in the September 16 to 20 between the average ten in four or five years will encounter a temperature below 17 ℃. Shanghai area in the past to discuss the latter part of the “Alice head” issue, only consider