Extinction of infrared radiation through continuous rain was measured with an infrared spectral radiometer at the wavelengths of atmospheric windows. Experimental results show that the extinction by rain is closely correlated with the precipitation rate. The quantitative relationship between extinction and precipitation rate was obtained from the experimental results. Extinciton by rain was also calculated using the Marshall Palmer particle size spectral distribution, and the calculated extinction coefficient coincides with the measurement results quite well when the forward scattering correction is considered.