From radar observations obtained at vertical incidence, a transient increase (5--10 dB on the average) of noise power in the Doppler spectra associated with lightning is analyzed. To explain this effect, three hypothesized causes are tested: (1) a sudden increase in echo power caused by lightning within the Fourier analysis window, (2) refractive index discontinuity, and (3) drop oscillation; refractive index discontinuity and drop oscillation are both produced by a lightning shock wave propagating through the sampling volume. The data analysis strongly suggests that the refractive index discontinuity is mainly responsible for the transient effect. ¿American Geophysical Union 1987