A uniformity index for the axis ratios (U_ar) derived from dual polarization weather radar data is proposed for raindrop area identification and analysis. The derivation of this new parameter is based on radar scattering simulations and assumptions. U_ar is between 0 and 1 and can be calculated from the differential reflectivity (Z_DR) and the copolar correlation coefficient (ρ_hv), which reflects the uniformity of the axis ratio (r) of the particle group. For raindrops, U_ar is close to 1 under ideal conditions, but is clearly different from that of ice particles whose value is close to 0. Studies conducted during two convective weather events observed by X-band and S-band radar are presented to show the U_ar features. In convective areas, high U_ar presents a U-shaped vertical structure. One branch corresponds to the Z_DR column, while the other branch is located at the rear of the convective cloud zone and is lower in altitude, representing the process of ice particles melting into raindrops and then being transported upward by a strong updraft. In stratiform cloud areas, a more than 95% overall identification ratio is obtained when the threshold of U_ar is set to 0.2~0.3 for discriminating rain layers.