1. A method, comprising: storing a set of references images without rain and spanning a plurality of different light conditions; capturing, using a camera, an image of a scene with rain; selecting a reference image from the set of reference images based on the light condition of the captured image; performing an arithmetic subtraction image processing operation between the captured image and the reference image to generate a subtraction image; and estimating an amount of rain in the subtraction image based on previously calibrated values.