关键词: 无降水日数, 干期, 多年变化, 年生长, 干旱特征, 距平, 天干, 十年滑动平均曲线, 生长季, 功率谱分析

Abstract: In this paper, the rainy and clear days observations and the instrumental records of Beijing are adopted to reconstruct the series of no rain days from 1724 to 1979. Two indices-no rain days and its duration are used to discuss the characteristics of drought and its long-term variation could be summarized as follows:1. The spring drought is one of the most serious damage to the crops of Beijing, 晈hich is characterized by frequent occurances and long duration of no rain days.2. The probability of continuous dry damage in successive seasons appears less than 10%. And this probability for three successive seasons (from spring to autumn) is only less than 3.4%.3. The non-rain days in the growing period was tending to decrease in Beijing during the last 256 years. Especially in the recent 30 years (1944-1979) this area appears to be passing through a phase of abnonnaly defficient rainfall. And from 1968 onwards the no rain days still possessed a negative deviation.4. It is shown that the no rain days possesses a fluctuation of hundred years (with cycles of about 170 and 85 years) from April to October.