中文摘要：

利用1975-2004年北京13个站的夏季降水资料，对30年来北京城区与郊区降水的时空变化趋势进行了对比分析，得出以下主要结论：1）无论是城区站还是郊区站，北京的夏季降水量均呈明显下降趋势，且城区站的夏季降水量总体上要小于郊区站。2）从大兴、海淀和昌平3站夏季降水量的时间变化特征来看，位于城区盛行风向下风向的昌平下降趋势最不明显，在一定程度上表征了城市化对北京夏季降水的影响。3）地形仍然是决定北京地区降水分布的主导因素，但降水高值区存在向西南城区方向延伸或移动的趋势，而城市化可能是造成这种变化的一个原因。

英文摘要：

Based on the analysis of summer precipitation data from 13 weather stations in Beijing area, a comparison of the summer precipitation evolution tendencies between the urban and rural region of Beijing is performed over a period of 30 years （1975 to 2004）. Some conclusions are drawn in the light of the analysis： 1） The summer precipitations in both rural and urban area had a descending tendency, and the precipitations in urban area were generally less than those in rural area over the 30 years. 2） The comparison among the summer precipitations at Daxing, Haidian and Changping weather stations shows that the descending rate of the summer precipitation at Changping station which is located in the downwind direction to the urban area is the slightest among the three ones, which can be taken as a token of the urbanization effect on the precipitation in Beijing. 3） Over the 30 years, the topography is still the determinative factor controlling the distribution of the precipitation in Beijing area, however, the zone with relatively abundant precipitation was moving or stretching toward the urban area over the 30 years. The impact of the rapid urbanization over the 30 years is a possible reason leading to the variation of the distribution of precipitation in Beijing area.