The entrainment process and its resultant effects on the microphysics and dynamics within cumuli are not yet clearly understood. This research was undertaken to discover the role which cloud top plays in the entrainment process and to determine whether observed downdraft magnitudes could be explained on the basis of evaporative cooling. An instrumented King Air research aircraft was used to acquire thermodynamic, microphysical and dynamical data within and near cumulus clouds of the high plains and midwestern United States. Temperature and liquid water content measurements made within the clouds were used to discover the source for any entrained air via a thermodynamic treatment initially pursued by Dufour (1956), and later expanded by Paluch (1979) and Betts (1982). The entrainment source regions found were either at or above the aircraft sampling level in 78 of the 87 cases examined. It was found that the environment within 20 mb of cloud top was the source for entrained air in these 78 cases. ... Abstract The entrainment process and its resultant effects on the microphysics and dynamics within cumuli are not yet clearly understood. This research was undertaken to discover the role which cloud top plays in the entrainment process and to determine whether observed downdraft magnitudes could be explained on the basis of evaporative cooling. An instrumented King Air research aircraft was used to acquire thermodynamic, microphysical and dynamical data within and near cumulus clouds of the high plains and midwestern United States. Temperature and liquid water content measurements made within the clouds were used to discover the source for any entrained air via a thermodynamic treatment initially pursued by Dufour (1956), and later expanded by Paluch (1979) and Betts (1982). The entrainment source regions found were either at or above the aircraft sampling level in 78 of the 87 cases examined. It was found that the environment within 20 mb of cloud top was the source for entrained air in these 78 cases. ...