In the visible and near IR, absorption is negligible so that the atmospheric extinction can be derived by atmospheric scattering which is mainly contributed by fog droplet, rain droplet, another types of droplet and small articles. The forward-scattering visibility meter (FVM) works by illuminating with near IR light a small sample volume of about 100 mL of air and measuring the intensity scattered in the angular range of 30 to 36 degrees. The scattered intensity is proportional to the extinction coefficient regardless of the article size distribution and after wavelength calibration. The ratio of scattered signal to extinction coefficient of fog and haze can be achieved by comparative test of FVM outputs and manual observations. Nevertheless, as a result of the application of the measurement during rain with the ratio of fog and haze, an unacceptable error is raised. To obtain an accuracy extinction measurement during rain, an appropriated ratio of scattered signal to extinction coefficient of rain would be found. The calculation for different size distributions of fog and rain with Mie theory has been made in this paper. And a comparison of extinction measurements made with two FVMs and manual observations during fog and rain has been made. The result shows that during rain the FVM extinction coefficient is from 20% to 60% greater than that of manual observations. This result can be used to define correction factors so that the FVM using forward-scattering near IR spectroscopy not only can be used to estimate extinction during fog and haze as well as during rain.