In this paper the monthly mean height fields at 500 hPa over the northern hemisphere are expanded. In the expanding the whole hemisphere can be divided into a northern and a southern latitudinal zones. Some statistical features of the main expansion coefficients and parameters of the harmonic waves are determined. Facts show that the flow fields and their seasonal variations are very different in the two zones. Results in this study suggest that in practice the expansion in the zones is better than that in the whole hemisphere.