A one dimensional cumulus cloud model has been developed to investigate the size distribution of hydrometeors at different heights and at different stages of cloud development. The model results show that the exponential distribution of droplet radius that was employed by many investigators can misinterpret the cloud dynamic fields during the life cycle of cumulus cloud. The results also show the bimodal distribution of cloud water mixing ratios at 25 $mu$m and 645 $mu$m in radius at 45 minutes of simulation time. The developed model can be applied in future on the parameterization of cloud microphysical processes and air pollution models.