Integral over the backscatter cross-section of the particles in a pulse volume. For particles small compared to the wavelength the scatter cross-section is D⁶, where D is the diameter of the particle. Radars are calibrated in the way to give directly (assuming the dielectric constant of water) the reflectivity factor from the received backscattered energy. Units for the reflectivity factor are mm⁶ m^-3 or the logarithmic value of this in dBZ.