The areal rainfall over the target areas for two different precipitation processes in Beijing are calculated with the data from climate stations only or from both climate stations and automatic precipitation stations by using the arithmetic and Thiessen polygon methods respectively. The results show that the Thiessen polygon method is advantageous in analyzing different kinds of precipitation events. The areal rainfall calculated with the data from both climate stations and automatic precipitation stations is much more accurate than that from climate stations only. For the assessment of precipitation enhancement effectiveness, a statistical correction method is proposed to calculate more accurately the areal rainfall over the target areas (including the control area) by using the historical rainfall data, which is highly important for improving the precision in the assessment of precipitation enhancement effectiveness.