The effects of warm rain on optical properties of clouds in the visible/near‐infrared (VNIR) and passive microwave (PMW) are studied using a simple conceptual cloud model. It is shown that the combined use of PMW and VNIR observations allows for the detection of precipitation and the derivation of rainwater path utilizing the different physical information content of the two observation types. Various potential error sources are studied and one month of combined geostationary visible/near infrared and Advanced Microwave Scanning Radiometer‐EOS (AMSR‐E) passive microwave observations off the coast of South Africa are evaluated using the proposed approach. Comparisons with CloudSat radar reflectivities are used for an independent assessment. A gradual increase in retrieved rainwater path with column maximum radar reflectivity is found for reflectivity values larger than −10 dBz. For monthly mean values at 1 × 1 degree resolution, rainwater path is correlated with in‐cloud liquid water path (R2 = 0.50). The strongest correlation (R2 = 0.69) exists between rainwater path and the inverse of cloud droplet number concentration (N). This finding is consistent with other studies supporting a 1/N dependency of precipitation intensity on cloud droplet number concentration in warm clouds.