A model of cumulus clouds is presented that combines the vertical equation of motion, the equation of mass continuity, the first law of thermodynamics, and the following cloud microphysical processes: condensation of water vapor to produce cloud droplets, conversion of cloud droplets to raindrops, glaciation, sublimation of water vapor, melting of ice crystals, evaporation of cloud droplets, evaporation of raindrops, evaporation of ice crystals, and evaporation of melting ice crystals. The conversion and glaciation processes are parameterized and the drag force is assumed to be provided by the weight of hydrometeors. The result of time integration of the model shows that, with the inclusion of the microphysical processes, some aspects of the three stages of the life cycle of a cumulus cloud as depicted by Byers and Braham in 1949 (developing stage, mature stage, and decaying stage) are simulated qualitatively by the model. The model also shows that the rate of conversion from cloud droplets to raindrops is important in determining the duration of the life cycle of a thunder-storm cell. This is exemplified in a case with a small rate of conversion where a thunderstorm cell is maintained in a steady state despite the drag force due to a large number of hydrometeors. Also investigated is the relative importance of various microphysical processes in determining dynamic as well as thermodynamic behavior of a cloud during the entire life cycle.