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The effectiveness evaluation of precipitation enhancement is made of 12 aircraft cloud seeding operations over the central Hebai Province by means of such evaluation methods as the double contrast based on regional rainfall tendencies, the correlative regression analysis based on regional rainfall tendencies, the covariable multiple regression analysis based on regional rainfall tendencies, the floating control historical regression method, the ClusterAnalysis〖CD*2〗based Floating Control historical regression Method (CAFCM) using rainfall as a covariant, and the CAFCM using rainfall and atmospheric precipitable water as covariants, all of which show greater than 15% or higher relative enhancement of rainfall. The comparison between CAFCM and the other evaluation methods indicates that the CAFCM using rainfall and atmospheric precipitable water as covariants is better than the others, because it adopts cluster analysis, which improves greatly the rainfall correlativity between control and target areas, and increases the accuracy of natural rainfall estimations over the cloud seeding areas by using atmospheric precipitable water as a covariant.