A new scheme is proposed for determining the standard deviationof Doppler velocity in the static atmosphere based on the radar reflec-tivity factor Z from the conventional weather radar and the rainfallrate I from the ordinary rain gauge. The raindrop size distribution indifferent types of precipitation is calculated by this method. Compari-son is made between the result using this method and that using thedefinition equation under the Rayleigh scattering condition. The averagerelative errors are found within 15-30%.