The rate of coalescence of cloud water to rain is estimated using new global satellite observations. The differing sensitivity of the passive measurements of reflected sunlight from MODIS and the cloud radar measurements of CloudSat to the sizes of cloud water particles in low, warm clouds is exploited for this purpose. The method integrates CloudSat radar reflectivity profiles with the MODIS cloud optical depth and effective radius matched to the radar footprint. The thresholds applied to the

 are chosen to restrict the mean cloud layer reflectivities (Z) observations to the incipient stages of warm rain development. MODIS optical property data provide cloud droplet liquid water path information. These different types of information, when combined, provide a means for estimating the rate of coalescence and hint at a time scale of the coalescence process that varies between about 26 minutes to 3.0 hours.