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Expansion of Chebyshev Fourier mixed polynomial is conducted by means of the NECP 500 hPa geopotential height data from 1981 to 1990 without rain enhancement operation and the same data in 1996 and 1997 with rain enhancement operation in Jilin Province to determine the minimum correlation between the two element fields. Hereby, case studies of different weather patterns in the two years are made and the percentages of rain enhancement effectiveness of 23 seeding flights are given. The results indicate that when the correlation between the two element fields reaches the minimum, the patterns of the two element fields are highly similar and the rainfall over the seeding area is mostly higher obviously; generally speaking, the rain enhancement operations have positive effect on rainfall.